CHAPTER XXII.

=264. Railroad Signalling.=—The birth of the art of railroad signalling was probably coexistent with that of the railroad. At the very outset of the movement of railroad trains it became imperative to insure to a given train the sole use of the single track at schedule periods. Both head-to-head and rear-end collisions were liable to occur on main tracks, as well as false meetings at branches and cross-overs.

=265. The Pilot Guard.=—One of the earliest if not the earliest of systematic procedures in England to accomplish the safe use of a railroad track involved the employment of the “pilot guard” on single-track roads. The pilot was an employé whose duty it was to accompany every train over a stated section of the line. The authority to start trains was lodged in him. When it became necessary to start two or three trains from the same point and in the same direction, it was also his duty to issue to each train conductor what was called a pilot ticket, equivalent to a modern train order to run the train over the section under his control. In that case he was obliged to accompany the last train to the other end of his section, and no more trains could move over that section in the same direction until his return to his first station. As no train could pass over the section without either him or his pilot ticket, it is clear that the system could prevent head-to-head collisions, but in itself it is not sufficient to eliminate rear-end collisions. This system is still employed in Great Britain on some short branch lines.

=266. The Train-Staff.=—Another method nearly as old as the preceding is that of the train-staff, used in an improved form at the present time on some single-track roads. No train under this system can pass over any given section of the line unless it carries the staff belonging to that section, the staff being a piece of wood or metal 1 to 1¼ inches in diameter and 18 to 20 inches long. In order to cover the case of two or more trains starting in the same direction at one end of a section before running a train in the opposite direction, tickets were issued, the staff being taken by the last train. The proper operation of this method, like that of the preceding, would prevent head-to-head collisions, but is not sufficient in itself to prevent one train running into the rear of another while both are proceeding in the same direction in the same section.

=267. First Basis of Railroad Signalling.=—These and other similar systems answered fairly well the more simple requirements of early railroad operation. Strictly speaking they are not methods of signalling, although it may be said that each train is a signal in itself. With the development of railroad business it was found that other methods better adapted to a more efficient and rapid movement of trains were imperative. It was in response to the advancing requirements of the railroad business that the first approach to what is now so well known as the block system of signalling was made in 1842. An English engineer, subsequently, Sir W. F. Cooke, stated the following sound principles as to the basis of efficient railroad signalling:

“Every point of a line is a dangerous point which ought to be covered by signals. The whole distance ought to be divided into sections, and at the end as well as at the beginning of them there ought to be a signal, by means of which the entrance to the section is open to each train when we are sure that it is free. As these sections are too long to be worked by a traction rod, they ought to be worked by electricity.”

The main features of railroad signalling, as thus set forth, have continued to characterize the development of the block system from that early day to the present. The electrical application to which reference is made in the preceding quotation was that of the needle, which by its varying position could indicate either “line clear” or “line blocked.” In 1851 electric bells were used in railroad signalling on the Southeastern Railway of England. Various other developments were completed from time to time in Great Britain until the Sykes system of block signalling was patented in 1875. One of the main features of the system, and perhaps the most prominent, was the control of the track signals at the entrance end of the block by the signalman at the advance end. He exerted this control by electrically operated locks. About 1876 the Pennsylvania Railroad introduced the block system into the United States, which has since been greatly developed in a number of different forms, and its use has been widely extended over many if not most of the great railroad systems of the country. It is not only used for the movement of trains, but also for the protection of such special danger-points as switches, cross-overs, junctions, drawbridges, heavy descending grades, sharp curves, and other points needing the protection which a well-designed block system affords.

=268. Code of American Railway Association.=—The code of the American Railway Association gives the following definitions among others pertaining to the block system:

_Block._—A length of track of defined limits, the use of which by trains is controlled by block signals.

_Block Station._—The office from which block signals are operated.

_Block Signal._—A fixed signal controlling the use of a block.

_Home Block Signal._—A fixed signal at the entrance of a block to control trains in entering and using said block.

_Distant Block Signal._—A fixed signal of distinctive character used in connection with a home block signal to regulate the approach thereto.

_Advance Block Signal._—A fixed signal placed in advance of a home block signal to provide a supplementary block between the home block signal and the advance block signal.

_Block System._—A series of consecutive blocks controlled by block signals.

_Telegraph Block System._—One in which the signals are operated manually upon telegraphic information.

_Controlled-Manual Block System._—One in which the signals are operated manually, and by its construction requires the co-operation of a signalman at both ends of the block to display a clear signal.

_Automatic Block System._—One in which the signals are operated by electric, pneumatic, or other agency, actuated by a train or by certain conditions affecting the use of a block.

=268a. The Block.=—It is seen by these definitions that what may be called the unit in railroad signalling is the “block”; it may be of almost any length from a few hundred feet to 6 or 8 miles, or even more. On a single-track railroad it may evidently extend from one side track or passing-place to another. Over portions of lines carrying heavy traffic it may be a half-mile or less. The length of block will depend, then, upon the intensity and kind of traffic, the physical features of the line, such as curvature, grade, sidings, cross-overs, and other similar features, the location, whether in cities, towns, or open country, as well as upon other elements affecting conditions of operation which it is desirable to attain.

=269. Three Classes of Railroad Signals—The Disc.=—The signals used in railroad operation may mainly be divided into three classes: semaphores, banners, and discs. In general they may convey information by form, position, and color. The disc is used by causing it to appear and disappear before an aperture, usually a little larger than itself, in a case standing perhaps 10 or 12 feet high alongside the track, and is admirably typified in the Hall electric signal. On account of its shape, the case in which the disc is operated is frequently called the banjo, as it is quite similar in shape to that musical instrument placed in a vertical position, the key end resting on the ground.

=270. The Banner Signal.=—The banner signal is usually operated by rotation about a vertical axis, frequently in connection with switches. Its full face painted red, exposed with its plane at right angles to the track, indicates “danger” or “stop.” With its face turned parallel to the track, showing only its edge to approaching trains, a “clear” line or “safety” is indicated.

In the present development of railroad signalling the banner and disc patterns have a comparatively limited application, although, on the whole, they are largely used. The banner signal is mostly employed in the manual operation of switches, turn-outs, and cross-overs, and for other local purposes, particularly on lines of light traffic.

[Illustration: FIG. 10.—Semaphore Signals.]

=271. The Semaphore.=—The semaphore is now mainly used in connection with block signalling. Like many other appliances in railroad signalling it was first used in England, by Mr. C. H. Gregory, about 1841. Its name is derived from the combination of two Greek words signifying a sign-bearer. It consists of a post varying in height from about 3 to 35 or 40 feet, carrying an arm at its top from 3 to 5 feet long, pivoted within a foot or 18 inches of one end, the long end suitably shaped and painted and the other arranged with a lens so that when operated at night in connection with a lamp it may exhibit a properly colored light. The post of the semaphore is placed alongside the track so as to be on the right-hand side of an approaching train, the long arm rising and falling as a signal away from the track and in a plane at right angles to it. The other arm of the semaphore signal may be connected by wires or rods and light chains running over pulleys with suitable levers and weights operated either in a near-by signal cabin or by a signalman stationed near the semaphore itself; or it may be operated by electric or pneumatic power, as in many of the later installations. The semaphore may, therefore, be operated at the post or by suitable appliances at a distance.

[Illustration: Semaphore on Pennsylvania Railroad.]

=272. Colors for Signalling.=—The colors used either for painted signals for daylight exposure or for coloring lenses for night signalling are red, white, and green, as ordinarily employed in this country; red signifying “danger” or “stop,” white signifying “safety” or “clear track,” and green signifying “caution” or “proceed with train under control,” indicating that a train may go forward cautiously, expecting to find an obstruction or occupied track. In England green is largely employed to indicate “safety” or “clear track,” on the ground that a white light is so similar to any other in its vicinity that the latter may too easily be mistaken for a signal. While there is some diversity of views in this country on that point, the consensus of engineering opinion seems to favor the retention of the white for the track safety signal.

=273. Indications of the Semaphore.=—It is evident that a semaphore affords facilities of form, position, and color in its use for the purpose of signalling. The horizontal position is the most striking for the semaphore arm, as it then extends at right angles to the post and to the right or away from the track; this position is, therefore, taken to indicate “danger” or “stop.” No train may, therefore, proceed against a horizontal semaphore arm.

It might at first sight appear that the vertical position of the semaphore arm close against the post could be taken to indicate “safety” or “clear track” or “proceed,” but experience has shown that such a position may be injudicious, except under special conditions where it has lately been employed to make that indication. If the semaphore arm should be knocked or blown from the ordinary post, the engineman of an approaching train probably would not be able to detect the actual condition of things and might accept the appearance of the semaphore as indicating a clear line, thus justifying himself in proceeding at full speed, while the signalman in his cabin might have placed the signal at “danger.” A position of the semaphore arm, therefore, at an angle of 65° or 70° below the horizontal is usually taken as a safety signal. This position is in marked contrast to the horizontal arm and at the same time makes the absence of the semaphore arm impossible without immediate detection from an approaching locomotive. After dark the semaphore in a position of danger exhibits a red light through the lens in its short arm when the long arm is at the “danger” position or horizontal. Similarly, when the long arm is in the safety position a white light is exhibited through the lens in the shorter arm, so that the respective conditions of clear or obstructed track are made evident to the engineman as well by night as by day on his approach to the semaphore.

In some of the latest signal work three positions of the semaphore arm on one post, known as three-position block signalling, have been employed. In this system a special post, frequently on a signal bridge over the track, permits the vertical position of the semaphore arm to indicate “clear track,” while the diagonal or inclined position below the horizontal indicates “caution.” In the Mozier three-position signal a diagonal or inclined position above the horizontal indicates “caution” an addition to the two usual positions of “stop” and “clear.”

These are the elements, so to speak, of railroad signalling at the present day. They are combined with various appliances and in various sequences, so as to express all the varied conditions of the track structure which affect the operation of the road or the movement of trains upon it. These combinations and the appliances employed in them are more or less involved in their principal features and complicated in their details, although the main principles and salient points are simple and may easily be exhibited as to their mode of operation and general results. In this treatment of the subject it will only be possible to accomplish these general purposes without attempting to set forth the mechanical details by which the main purposes of railroad signalling are accomplished.

=274. General Character of Block System.=—It is evident from what has already been stated that the block system of signalling involves the use of fixed signals located so as to convey promptly to approaching trains certain information as to the condition of points of danger approached. Furthermore, this system of signals is designed and operated on the assumption that every point is to be considered as a danger-point until information is given that a condition of safety exists. The usual position of signals, or what may be called the normal position, is that of “danger,” and no position of “safety” is to be given to any signal except to permit a train to pass into a block whose condition of safety or clear track is absolutely assured. These are the ground principles on which the signal systems to be considered are designed and operated, although there are some conditions under which the normal signal position may be that of safety.

=275. Block Systems in Use.=—The block systems now in general use are:

The Manual, in which the signals at each end of each block are wholly controlled and operated by the signalman at each signal point.

The Controlled-Manual, in which the signals at the entrance to each block are controlled either electrically or in some other manner by the signalman at the other extremity of that block, but are operated subject to that control by the signalman at the entrance of the block.

The Auto-Manual, in which the signals are generally operated and controlled as in the Manual or Controlled-Manual, except that they are automatically returned to the danger position as the rear car of a moving train passes them.

The Automatic, in which the operation of the signals is wholly automatic and generally by electricity, or by a combination of electric and pneumatic mechanism. In this system no signalmen are required.

The Machine, which is a controlled block system for single-track operation and in which machines operated electrically with detachable parts, as staffs, are employed in connection with other fixed signals alongside the track.

The main features of these various systems of blocking are, in respect to their signalling, the same, but the means for actuating or manipulating the signals and the conditions under which moving trains receive the necessary instructions are different. They all have the same main objects in view of improving railroad operation by enhancing both safety and facility of train movement.

“Absolute” blocking is that system of block signalling which absolutely prevents one train passing into a block until the preceding train is entirely out of it, or, in other words, until the block is absolutely clear.

“Permissive” blocking is, strictly speaking, the violation of the true block system of signalling, since under it a train may under certain precautionary conditions enter a block before the preceding train has passed out of it.

=276. Locations of Signals.=—In proceeding to locate signals along a railroad line it is imperative to recognize the preceding purposes as controlling motives. Signals must be seen readily and clearly in order to be of the greatest service to the enginemen of approaching trains, and their positions must be selected with that end in view. Locations of switches, cross-overs, junctions, and other similar track features will control the locations of the signals which are to protect them. The main or home signal in these special cases may usually be placed from 50 to 200 feet from the point which is to be governed, the so-called “distant” signal being placed about 2000 feet for level track back of the main or home signal.

=277. Home, Distant, and Advance Signals.=—A complete system of signals employed in blocking includes first of all the so-called “home” signal at each extremity of a block, then at a distance of 2000 to 2500 feet back from the home signal is placed the “distant” signal. The latter is thus approached and passed before reaching the home signal. On the other side of the “home” signal at least a maximum train length into a block about to be entered by a moving train is placed the “advance” signal. The distance of the advance signal from the home signal may be 1500 to 2500 feet. As a moving train approaches the end of a block it first meets the distant signal, the purpose of which is to indicate what the engineman may expect to find at the home signal. If the distant signal is in the danger position, he will pass it with caution and place his train under control so as to be able to stop at the home signal. If he finds the distant signal in a safety position, indicating the same position of the home signal, he may approach the latter without reducing speed, confident that the next section is clear and ready for him. The advance signal forms a kind of secondary or supplementary block into which the train, under certain conditions, may enter when the block in which it is found is obstructed, but no train may pass the advance signal unless the entire block is clear except when, under permissive working, the train proceeds with caution, expecting to find the track either obstructed or occupied. This group of three signals—the distant, the home, and the advance—taken in the order in which the moving train finds them, is located at each extremity of the block. Although the home signal is said to control the movement of trains in a block at the entrance to which it is found, as a matter of fact it appears that the advance signal in the final event holds that control.

=278. Typical Working of Auto-Controlled Manual System.=—The mode of employing these signals can be illustrated in a typical way by the diagrams, Figs, 11, 12, and 13, which exhibit in a skeleton manner Pattenall’s improved Sykes system which belongs to the Auto-Controlled Manual class. In these figures the end of block 1, the whole of blocks 2 and 3, and the beginning of block 4 are shown. Stations _A_, _B_, and _C_ indicate the extremities of blocks. The signals _S_, _Sʹ_, and _S″_ are the home signals, while _D_, _Dʹ_, and _D″_ indicate distant signals, and _A_, _Aʹ_, and _A″_ advance signals. As the diagrams indicate, the stretch of double-track road is represented with east- and west-bound tracks. In order to simplify the diagrams, signals and stations are shown for one track only; they would simply be duplicated for the other track. The signal cabin is supposed to be located at each station, and at that cabin are found the levers and other appliances for working the signals operated there, the signals themselves being exposed alongside the track. In each signal cabin there is an indicator, as shown at _I_, _Iʹ_, and _I″_. On the face of each indicator there are two slots, shown opposite the lines _E_ and _F_. In the upper of these slots appears either the word “Clear” or “Blocked.” In the lower slot appears either the word “Passed” or “On.” The significance of these words will appear presently. On this indicator face at _P_, _Pʹ_, and _P″_ are located electric push-buttons called plungers. The operation of the levers indicated at _L_, the counterweights _d_, and the locking detail _l_ are evident from an inspection of the figure, and need no special explanation. It is only necessary to state that the locking-device _l_ holds the bar _bc_ until it is released at the proper time, and that the counterweight may then return the lever from its extreme leftward position to that at the extreme right, at the same time placing the semaphore arm _S_ in the position of danger. It is particularly important to bear in mind this last observation. The counterweight is the feature of the system which always holds the semaphore arm in the position of danger, making that its normal position, except when it is put to safety for the passing of a train.

[Illustration: FIG. 11.]

[Illustration: FIG. 12.]

[Illustration: FIG. 13.]

If a westward train is represented in Fig. 11 at _T_ as approaching station _A_ to enter the block 2, both the distant signal _D_ and the home signal _S_ being at danger, the system is so arranged that the signalman at station _A_ cannot change those signals, i.e., to a position of safety, until the signalman at station _B_ permits him to do so. If the signalman at station _A_ desires to open block 2 for the entrance of the train _T_, he asks the signalman at station _B_ by wire to release the lock _l_ to enable him to do so. If there is no train in block 2, the signalman at station _B_ pushes the button _P′_ or “plunges” it. This raises the lock _l_ at station _A_ and the signalman immediately pulls the lever _L_ to its extreme leftward position, throwing both the signals _S_ and _D_ to the position of safety or clear, indicated by the dotted lines at _S_². At the same time the indicator _E_ at station _A_ shows the words “Clear to _B_,” while the slot _Fʹ_ at _B_ shows the words “On from _A_.” The signals at stations _B_ and _C_ are supposed to be in their normal position of danger, and the indicator _E′_ at station _B_ shows the words “Blocked to _C_.” The home and distant signals _Sʹ_ and _Dʹ_ are now at danger, but the train _T_ may enter block 2 and proceeds to do so, it being remembered that the signalman at station _A_ cannot move the lever _L_, as it has passed out of his control; not even the signalman at station _B_ can give him power to do so. The train _T_ now passes station _A_ into block 2. As the last car passes over the point _G_ its wheels strike what is called a track-treadle, an appliance having electrical connection with the lock _l_. The effect of the wheels of the last car of the train passing over the treadle at _G_ is to release lock _l_, enabling the signalman at station _A_ immediately to raise the arms _S_ and _D_ to the position of danger. It is to be observed that he cannot do this until the entire train has passed into block 2; nor, since his plunger is locked by the same treadle at _G_, can he signal “Safety” or “Clear” to the entrance of block 1. Hence no train can enter block 1 to collide with the rear end of the train just entering block 2. When the signalman at station _A_ has raised his signal _S_ to danger, it again passes out of his control, indeed out of both his control and that of the signalman at _B_, until the last car of the train passes over the treadle _Gʹ_ at the entrance of block 3.

The train has now passed into block 2 and is approaching station _B_. The signalman at _B_ asks _C_ by wire to release the lever _Lʹ_, and if block 3 is clear, _C_ plunges at _P″_.

_C_ then throws his lever _Lʹ_ so as to place the home and distant signals _Sʹ_ and _Dʹ_ at safety. The condition of things will then be shown by Fig. 12. As soon as the last car of the train has passed over the treadle at _Gʹ_ his lever _Lʹ_ will be released and he can then throw the lever to the danger position, raising the home and distant signals _Sʹ_ and _Dʹ_ to the horizontal. After the danger position is assumed by the home signal _Sʹ_, as well as the distant signal _Dʹ_, he has no power over them until the signalman at station _C_ confers it on him by plunging the button _P″_.

While the train has been in block 2, the indicator _Iʹ_ has shown “Blocked to _C_” and “Train on from _A_,” but as the train passes _B_ the indicator reads “Blocked to _C_” and “Train passed from _A_,” while the indicator _I″_ at _C_ reads “Blocked to _D_” and “Train on from _B_.” This condition of the signals and trains is shown by Fig. 13. Also, when the last car passes over the treadle _Gʹ_, but not till then, _B_ may permit _A_ to admit a train to enter block 2 should _A_ so desire. Finally, when the train approaches _C_, the signalman at that point asks _D_ to enable him to permit the train to enter block 4, and _C_ confers the power by plunging if that block is clear. Fig. 13 exhibits the corresponding signals at _C_.

This sequence of operations is typical of what takes place in this particular block signal system at the limits of every successive block, and differs only in details characteristic of this system from those which are performed in any other block signal system.

=279. General Results.=—It is seen first that no signalman can operate a signal until the condition in the block ahead of him is such as to make it proper for him to do so, and then he can only indicate what is necessary for the safe entrance of the train into that block. Furthermore, immediately on the passage of the train past his home signal he must put the latter to danger or the counterweight may do it for him, the train itself when in a safe position having conferred the requisite power upon him. The signalman at the advance end of the block always knows when the train is about to enter it, for he is obliged to give his permission for that entrance. His indicator shows this result, and will continue to show it until the train passes out of the block. It is to be observed that the upper openings marked _E_ on the indicator give information of the condition of the block in advance, while the lower openings give information of the block in the rear.

It is particularly important to notice that after the signalman at the advance end of a block has “plunged” his plunger remains locked and it cannot be released until the train admitted to the block covered by the plunger has completely passed out of that block, permitting the track-treadle at the entrance to the next block to unlock the plunger. This feature makes it impossible for one train to enter a block until the preceding train has passed out of it.

If the permissive system of using a block be employed, in which the train is permitted to enter that block before a preceding train leaves it, the treadle gives no protection against a rear-end collision with the first train. In such an exigency other devices must be used or the following train must proceed cautiously, expecting to find the track occupied.

=280. Distant Signals.=—Thus far the distant signals have been treated incidentally only. They may be operated concurrently with or independently of the home signal in such a way that if danger is indicated, the distant signal gives its indication prior to that of the home signal. In this manner protection is given to the rear of a train approaching a block against the home signal set at “danger.” After the obstruction is removed and the block cleared, the home signal is set at “safety” before the distant signal is cleared.

=281. Function of Advance Signals.=—The advance signals are used when for any purpose it is desired to form a short block in a regular block. If, for instance, block 3 in Fig. 11 were obstructed by a train stopped by some failure of a locomotive detail, a train approaching station _B_ in section 2 against the home signal _Sʹ_ set at “danger” would be obliged to stop before entering block 3. It might then be permitted to enter the latter block, to be stopped by the advance signal _Aʹ_ set at “danger” or under instructions to pass it cautiously, expecting to find the track obstructed. It is thus seen that the advance signal creates what may be called an emergency block, and in reality finally controls the movement of trains in the block in which it is located. It would never be cleared unless the home signal were first cleared, nor would it be set at “danger” unless the home signal gave the same indication.

The preceding operation of the block system of signalling controls the movement of trains along a double-track line.

[Illustration: FIG. 14.]

=282. Signalling at a Single-track Crossing.=—A somewhat similar sequence of signal operations controls train movements at a crossing, whether single- or double-track. Fig. 14 illustrates the use of signals required for the safe movement of trains at a single-track railroad crossing, which is supposed to be that of a north-and-south line crossing obliquely an east-and-west line. Precisely the same arrangement of signals operated in the same manner would be required if the crossing were at the angle of 90°. The signal cabin is placed, as shown, as near as practicable to the actual intersection of tracks. Trains may pass in either direction on either track, but in every case they would be governed by the signals at the right-hand side of the track as seen by the engineman. There will therefore be a set of signals on both sides of each track, each set governing the movement of trains in its own direction. Each home signal may be placed about 350 feet from the actual intersection, and each distant signal 1200 to 1500 feet from the home signal, or 1550 feet to 1800 feet from the intersection. Each advance signal must be at least as far in advance of the home signal as the maximum length of train, since it may be used to stop a train, the rear car of which should completely pass the home signal. In their normal positions every home signal should be set at “danger,” carrying with them the distant signals giving the same indication. The advance signals must also indicate “danger” with the home signal. No train can then pass the crossing until the home and distant signals indicate a clear line for it, the other signals at the crossing, except possibly the advance signal, being set at “danger.” If for any reason it is desired to hold the train after it is entirely free of the crossing, the advance signal would also indicate “danger.”

It is thus seen that if the signals are properly set and obeyed, it is impossible for two trains to attempt a crossing at the same time. It is not an uncommon occurrence, however, for an engineman to run his train against the danger signal, and in order to make it impossible for the train to reach the crossing even under these circumstances a derailing device is used. This derailing arrangement is shown in Fig. 14, about 300 feet from the crossing, although it may be placed from 300 to 500 feet from that point. Its purpose is to derail any train attempting to make the crossing against the danger signal. The operation of the derail is evident from the skeleton lines of the figure. When the home signal is at danger the movable part of the derailing device is at this point turned so as to catch the flanges of the wheels as they attempt to pass it. The train is thus thrown upon the cross-ties at such a distance from the crossing as will produce a stop before reaching it. When the home signal is at safety the derail operated with the signal is closed and the line is continuous. This combination of signals and derail coacting serves efficiently to prevent collisions at crossings, although trains may be occasionally derailed in accomplishing that end. The preceding explanations of the use of signals and derail apply to a train that may approach the crossing in either direction on either track, as is obvious from an inspection of the diagram itself.

=283. Signalling at a Double-track Crossing.=—In the case of a double-track crossing, the arrangement of signals and derails is precisely the same as for a single-track crossing, each set of signals shown in Fig. 15 covering one track. In other words, the line of single track is to take the place of each rail with its set of signals in that figure. There will be but four derails, one for each track only on the approach to the crossing. The working of the signals with the derails is precisely the same as has already been explained for the single-track crossing.

[Illustration: FIG. 15.]

[Illustration: FIG. 16.]

=284. Signalling for Double-track Junction and Cross-over.=—Fig. 16 represents a skeleton diagram of signals required for a junction of two double-track roads and a cross-over. This arrangement covers the use of switches. The location of signals and signal cabin as shown is self-explanatory, after what has already been stated in connection with single- and double-track crossings. It will be observed that the home signals for both the west-bound main and branch tracks are identical in location, and are shown by the solid double flag, the distant signal being shown by its notched end at a considerable distance back of the double home signal. It will, furthermore, be observed that at each home signal there is a derailing-switch interlocked, in the lock-and-block system presently to be explained, with the home signals operated simultaneously with them. If, therefore, an engineman attempts to run his train past a home signal set at danger, the result will be the derailment of his train, thus brought to rest before it can make any collision with another. It is obvious in this case that if the switches from the main to the branch tracks or at the extremities of the cross-over are worked independently, they must be operated directly in connection with the signals. For complete protection they should be interlocked with the signals so that it would be impossible to clear any signal without simultaneously setting the switches consistently with those signals. The diagram exhibits clearly the indications which must be made in order to effect any desired train movement at such a junction of tracks.

=285. General Observations.=—Similar arrangements of signals, derails, or switches must be made wherever switches, cross-overs, and junctions are found, the detailed variations of those signals and switches being made to meet the individual requirements of each local case. The combinations of switches and switch-signals frequently become very complicated in yards where the tracks are numerous and the combinations exceedingly varied, in order to meet the conditions created by the movement of trains into and out of the yard.

The preceding explanations are intended only to give a clear idea of the main features of signalling, in order to secure the highest degree of safety and facility in the movement of trains over a modern railroad. While they exhibit the external or apparent combinations of signals for that purpose, they do not touch in detail and scarcely in general upon the mechanical appliances found in the signal cabin and along the tracks required to accomplish the necessary signal movements. The considerations in detail of those appliances would cover extended examinations of purely mechanical, electrical, pneumatic, and electro-pneumatic combinations too involved to be set forth in any but the most extended and careful study. They have at the present time been brought to a wonderful degree of mechanical perfection and afford a field of most interesting and profitable study, into which, however, it is not possible in these general statements of the subject to enter.

[Illustration: FIG. 17.]

=286. Interlocking-machines.=—The earliest machine perfected for use in this department of railroad signalling was the Saxby and Farmer interlocking-machine, first brought out in England and subsequently introduced in this country between 1874 and 1876. This machine has been much improved since and has been widely used. Other interlocking-machines have also been devised and used in this country in connection with the most improved systems of signalling, until at the present time a high degree of mechanical excellence has been reached.

The interlocking-machine in what is called the lock-and-block system of signalling is designed to operate signals, or signals in connection with switches, derailing-points, or other dangerous track features, so as to make it impossible for a signalman to make a wrong combination, that is, a combination in which the signals will induce the engineman to run his train into danger. The signals and switches or other track details are so connected and interlocked with each other as to form certain desired combinations by the movement of designated levers in the signal cabin or tower. These combinations are predetermined in the design and connections of the appliances used, and they cannot be changed when once made except by design or by breakage of the parts; they cannot be deranged by any action of the signalman. He may delay trains by awkward or even wrong movement of levers, but he cannot actually clear his signals for the movement of a train without simultaneously giving that train a clear and safe track. As has been stated, he cannot organize an accident. Figs. 17 and 18 show banks or series of levers belonging to interlocking-machines. As is evident from these figures, the levers are numerous if the machine operates the switches and signals of a large yard, for the simple reason that a great many combinations must be made in order to meet the requirements of train movements in such a yard. The signalman, however, makes himself acquainted with the various combinations requisite for outgoing and incoming trains and the possible movements required for the shifting or hauling out of empty trains. He has before him diagrams showing in full the lever movements which must be made for the accomplishment of any or of all these movements, and he simply follows the directions of the diagrams and his instructions in the performance of his duty. He cannot derange the combinations, although he may be slow in reaching them. The locking-frame which compels him to make a clear track whenever his signals give a clear indication to the engineman lies below the lower end of the levers seen in the figures. The short arms of the levers carry tappets with notches in their edges into which fit pointed pieces of metal or dogs; the arrangement of these notches and dogs is such as to make the desired combinations and no others. It will be observed that a spring-latch handle projects from a point near the upper end of each lever where the latter is grasped in operating the machine. This spring-latch handle must be pressed close to the lever before the latter can be moved. The pressing of the spring-latch handle against the lever effects a suitable train of unlocking before which the lever cannot be moved and after which it is thrown over to the full limit and locked there. The desired combination for the movement of the train through any number of switches may require a similar movement of a number of levers, but the entire movement of that set, as required, must be completely effected before the signals are cleared, and when they are so cleared the right combination forming a clear track for the train, and that one only, is secured. These meagre and superficial statements indicate in a general way, however imperfectly, the ends attained in a modern interlocking-machine. They secure for railroad traffic as nearly as possible an absolutely safe track. They eliminate, as far as it is possible to do so, the inefficiency of human nature, the erratic, indifferent, or wilfully negligent features of human agency, and substitute therefor the certainty of efficient mechanical appliances. In some and perhaps many States grade crossings are required by statute to adopt measures that are equivalent to the most advanced lock-and-block system of signalling. So vast has become railroad traffic upon the great trunk lines of the country that it would be impossible to operate them at all without the perfected modern systems of railroad signalling. They constitute the means by which all train movements are controlled, and without such systems great modern railroads could not be operated.

[Illustration: FIG. 18.]

The swiftly moving “limited” express passenger trains, equipped with practically every luxury of modern life, speed their way so swiftly and smoothly over many hundreds of miles without the incident of an interruption, and in such a regular and matter-of-fact way, that the suggestion of an intricate system of signalling governing its movements is never thought of. Yet such a train moves not a yard over its track without the saving authority of its block signals. If the engineman were to neglect even for a mile the indication of the semaphore, he would place in fatal peril the safety of his train and of every life in it.

=287. Methods of Applying Power in Systems of Signalling.=—The mechanical appliances used in accomplishing these ends are among the most efficient in character and delicate yet certain in motive power which engineering science has yet produced. The electric circuit formed by the rails of the track plays a most important part, particularly in securing the safety of the rear of the train in making it absolutely certain whether even rear cars that may have broken away have either passed out of the block or are still in it. The electric circuit in one application or another was among the earliest means used in railroad signalling. Electric power is also used in connection with compressed air for the working of signals. Among the latest and perhaps the most advanced types of lock-and-block signalling is that which is actuated by low-pressure compressed air, the maximum pressure being 15 pounds only per square inch. The compressed air is supplied by a simple compressor, and it is communicated from the signal cabin to the most remote signal or switch by pipes and suitable cylinders fitted with pistons controlled by valves, thus effecting the final signal or switch movements. It has been successfully applied at the yard of the Grand Central Station in New York City and at many other similar points. In this connection it is interesting to observe that while the original Saxby and Farmer interlocking-machine was installed from England in this country, as has already been observed, about 1875, American engineers have within a year reciprocated the favor by furnishing and putting in place most successfully in one of the great railroad yards of London the first low-pressure pneumatic lock-and-block system[8] found in Great Britain.

[8] By Standard Railroad Signal Company of Troy, N. Y.

=288. Train-staff Signalling.=—The lock-and-block system gives the highest degree of security attainable at the present time for double-track railroad traffic, but the simpler character of the single-track railroad business can be advantageously controlled by a somewhat simpler and less expensive system, which is a modification of the old train-staff method. It is one of the “machine” methods of signalling. The type which has been used widely in England, Australia and India, and to some extent in this country is called the Webb and Thompson train-staff machine, shown in Fig. 19. It will be observed that the machine contains ten staffs (18 to 20 inches long and 1 to 1¼ inches in diameter), but as many as fifteen are sometimes used. These staffs can be removed from the machine at one end of a section of the road at which a train is to enter, only by permission from the operator at the farther end of the section. If the station at the entrance to that section is called _A_, and the station at the farther end _X_, the following description of the operation of the instrument is given by Mr. Charles Hansel in a very concise and excellent manner:

[Illustration: FIG. 19.—Webb and Thompson Train-staff Machine.]

“When a train is ready to move from _A_ to _X_ the operator at _A_ presses down the lever which is seen at the bottom of the right-hand dial, sounding one bell at _X_, which is for the purpose of calling the attention of the operator at _X_ to the fact that _A_ desires to send a train forward. The operator at _X_ acknowledges the call by pressing the lever on his instrument, sounding a bell in the tower at _A_. The operator at _A_ then asks permission from _X_ to withdraw staff by pressing down the lever before mentioned three times, giving three rings on the bell at _X_, and immediately turns his right-hand pointer to the left, leaving it in the horizontal position pointing to the words ‘For staff,’ indicating that he desires operator at _X_ to release his instrument so that he can take a staff or train order from it. If there is no train or any portion of a train between _A_ and _X_, the holding down of the lever at _X_ closes the circuit in the lock magnets at _A_, which enables the operator at _A_ to withdraw a staff. As soon as this staff is removed from _A_, _A_ turns the left-hand pointer to the words ‘Staff out,’ and in removing this staff from the instrument _A_ the galvanometer needle which is seen in the centre of the instrument between the two dials vibrates, indicating to the operator at _X_ that _A_ has withdrawn his staff. _X_ then releases the lever which he has held down in order that _A_ might withdraw a staff and turns his left-hand indicator to ‘Staff out,’ and with this position of the instrument a staff cannot be withdrawn from either one.

“The first method of delivering this staff to the engineer as a train order was to place it in a staff-crane, which crane was located on the platform outside of the block station. With the staff in this position it has been found in actual practice that the engineman can pick it up while his train is running at a speed of 30 miles per hour. A second staff cannot be removed from _A_ nor a staff removed from _X_ until this staff which was taken by the engineman in going from _A_ to _X_ is placed in the staff instrument at _X_; consequently the delivering of a staff from _A_ to the engineman gives him absolute control of the section between _A_ and _X_.

“This train order staff also controls all switches leading from the main line between _A_ and _X_, for with the style of switch-stand which we have designed for the purpose the trainman cannot open the switch until he has secured the staff from the engineman and inserted it in the switch-stand, and as soon as he throws the switch-lever and opens the switch he fastens the train-staff in the switch-stand, and it cannot be removed until the switchman has closed and locked the switch for the main line. When this is done he may remove the train-staff and return it to the engineman. It will thus be seen that this train order, in the shape of a staff, gives the engineman absolute control over the section, and also insures that all switches from the main line are set properly before he can deliver the train-staff to the instrument at _X_.

“In order that the operator at _X_ may be assured that the entire train has passed his station, we may divide the staff in two and deliver one half to the engineman and the other half to the trainman on the caboose or rear end of the train, and it will be necessary for the operator at _X_ to have the two halves so that he may complete the staff in order to insert it into the staff instrument at _X_, as it is impossible to insert a portion of the staff; it must be entirely complete before it can be returned to the staff instrument.”

Instead of using the entire staff as a whole or in two parts, Mr. Hansel suggests that one or more rings on the body of the staff be removed from the latter and given to the engineman or other trainman to be placed upon a corresponding staff at the extreme end of the section. This would answer the purpose, for no staff can be inserted in a machine unless all the rings are in their proper positions. These rings can be taken up by a train moving at any speed from a suitable crane at any point alongside the track.

For a rapid movement of trains on a single-track railroad under this staff system an engineman must know before he approaches the end of the section whether the staff is ready for delivery to him. In order to accomplish that purpose the usual distant and home signals may readily be employed. The distant signal would show him what to expect, so that he would approach the entrance to the section either at full speed or with his train under control according to the indication. Similarly, electric circuits may be employed in connection with the staff or rings in the control of signals which it may be desired to employ.

The electric train-staff may also be used in a permissive block system, the section of the track between stations _A_ and _X_ constituting the block. In Fig. 19, showing the machine, a horizontal arm is seen to extend across its face and to the right. This is the permissive attachment which must be operated by the special staff shown on the left half of the machine about midway of its height. If it is desired to run two or three trains or two or three sections of the same train from _A_ before admitting a train at _X_ in the opposite direction, the operator at _A_ so advises the operator at _X_. The latter then permits _A_ to remove the special staff with which the extreme right-hand end of the permissive attachment is unlocked and a tablet taken out. This tablet is equivalent to a train order and is given to the train immediately starting from _A_. A second tablet is given in a similar manner to the second section or train, and a third to the third section. The last section of train or train itself starting from _A_ takes all the remaining tablets and the special staff for insertion in the machine at _X_. In this manner head-to-head collisions are prevented when a number of trains are passing through the block in the same direction before the entrance of a train in the opposite direction. This system has been found to work satisfactorily where it has been used in this country, although its use has been quite limited. Evidently, in itself, it is not sufficient to prevent rear-end collisions in a block between trains moving in the same direction. In order to avoid such collisions where a train falls behind its schedule time or for any reason is stopped in a block, prompt use must be made of rear flagmen or other means to stop or to control the movement of the first following train.

[Illustration: FIG. 20.]

The most improved form of high-speed train-staff machine is shown in Fig. 20, as made and installed by the Union Switch and Signal Company and used by a number of the largest railroad systems of the United States. In these machines the staffs are but a few ounces in weight.