nal counterweight and the signal rod, which connection, if removed when the signal is in the "clear" position, permits the signal to return to "danger" by force of gravity. The signal cannot again be placed at "safety" until the signal lever has been placed in its normal ("danger") position. This device, therefore, prevents a signalman, after having allowed a train to enter a block, from carelessly and prematurely releasing the signal at the rear station while a train is in the block.

To ensure absolute safety for the main track in a block, including the territory for at least 1,000 feet ahead of the home signal at the station in advance (or a greater distance on a down grade), the main track switches at such advance station (including those within the 1,000-foot limit), and all outlying switches in the block, must be electrically interlocked with the block station in the rear, so that the signals at the station in the rear, which control the entrance to the block, cannot be placed at "clear" when the main track is occupied, or is fouled by a train on a side track, or when a main track switch is thrown for a side movement, or a main track "back up" signal is set for a backward movement. This system applies only to the operation of trains under the absolute block system.

Trains may run up to, but not beyond, home or advance signals at "danger." They must not proceed until the clear signal or proper authority to pass the "danger" signal is given. When a distant signal is at "caution" trains may pass it under control, prepared to stop at the home signal. Home, advance and distant signals at "safety" may be passed without reducing schedule speed. A train that is to be held at an advance signal must first be brought to a stop at the home signal. It may then be allowed to proceed, under control, to the advance signal. The clearing of a home signal, in any case, gives authority to proceed to the advance signal only. To avoid the necessity of placing the home signal at "clear" for a train that is to be held at the advance signal, an auxiliary signal may be used. This is known as a "calling on" signal, and is a short semaphore blade fixed upon the home signal post below the home signal blade. When in the "safety" position this signal gives a train authority to pass the home signal at "danger" and proceed under control to the advance signal.

Automatic Blocking. Automatic block signals are operated through the agency of electricity or of a combination of electricity and compressed air, by the passage of trains into and out of the block. A track circuit is generally used in automatic blocking, for conveying the electric current to the signals. In arranging a track circuit, a gravity battery is commonly used, and is placed at the advance end of the block. A relay and the signal are placed at the other end. Each line of rails is connected with a pole of the battery. The current passes along the rails of one side of the track, through the relay, thence along the rails of the other side of the track, thus forming a circuit. The relay controls a local circuit which works an electro-magnet, which in the simple electric system actuates the signal, and in the electro-pneumatic system permits compressed air to actuate it. The electric

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power may be generated by a dynamo. The use of storage batteries in electro-pneumatic interlocking permits of a closer regularity of the quantity and pressure of the current, and results in considerable economy in operation.

Automatic block signals are operated under two plans: (1) the normal-safety plan, that in which the signals always show "clear" unless the block is obstructed either by a train or otherwise; (2) the normal-danger plan, that in which the signals are at "danger" at all times except when cleared by an approaching train, which can be done only when the block is clear of trains and other obstructions.

Automatic block signals are applicable to junctions, railroad crossings and drawbridges if manual interlocking is used. Separate signals are provided for the interlocking. The same rules apply to the block signals here as elsewhere. If the interlocking and block signals both show clear," trains may proceed with speed. At a junction, where the route diverges, the block signals will show "clear" when there is no train in the block ahead on the main line, and the interlocking signal is lowered for a train to continue on the main line, or when there is no train in the block ahead on the branch line and the interlocking signal for the branch is lowered.

The ordinary automatic block system provides protection for following movements, but for opposing movements it fails to meet the requirements of an ideal system for the reason that the block for opposing movements does not extend from siding to siding, an inherent defect in the system.

In the absolute permissive block system the block for opposing movements is from siding to siding and the signals governing entrance to this block are in all cases absolute or stopand-stay signals, hence the term "absolute» in the name of the 'system. For following movements the block between sidings is divided into two or more sub-blocks, as traffic conditions may require, and movements into these subblocks are governed by intermediate signals which may be absolute (stop-and-stay), or permissive (stop-and-proceed) signals. Ordinarily, intermediate signals are permissive or stop-andproceed signals as used on double track, hence the term "permissive" in the name of the system.

Absolute Permissive Block System.This system of automatic block signals is adapted to meet the exacting requirements of single-track signaling. On double track, train movements on the same track are in the same direction. The fundamental requisite of doubletrack signaling is to so arrange the signals as to provide a space interval between trains corresponding to a given time interval. For example, if the maximum traffic consists of trains operated at one-minute intervals and at a speed of 60 miles per hour, there would be a space interval between trains of one mile (less the train length). This space interval is divided into block sections so arranged as to permit trains to follow each other under adequate signal protection.

Ôn single track, train movements are in both directions. The fundamental requisites are two, one for following movements as in double track and the other for opposing movements. To accommodate following train

block system. Another requisite is that a reliable caution indication be displayed for each stop indication. This is a vital point at passing sidings where opposing trains approaching each other must have ample warning of a signal in the stop position.

The system is flexible and can be easily arranged to provide for the special requirements of any railroad. The chief advantages of the system are: (1) Opposing train movements are governed by absolute signals, the block extending from siding to siding; (2) following train

the ordinary system; (7) due to the protection against opposing trains "tonnage» signals, so called, may be used to eliminate the stoppage of tonnage trains on ascending grades where it would be difficult to start them; (8) signals are restored to normal position as soon as the block section is clear, regardless of any sequence of movements; (9) minimum number of signal appliances and line wires required.

Interlocking Signals. Movements in the reverse direction of traffic (or "backing-up" movements) are sometimes necessary and these

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must be guarded by separate signals provided for the purpose. The apparatus used at railroad crossings and junctions to ensure the proper display of the switch signals when the switches are changed from one position to another that is, set for a different route and to operate the switches and signals from a central point is known as an interlocking machine. The fundamental principle of the design of the interlocking machine is that when the switches and signals are set for any one route no switch or signal can be set for any conflicting route, or a route which would foul the route already "cleared." In this way collisions at junctions and switches are effectually prevented. The interlocking signals are not a part of the block system proper, but, as already stated, they should be arranged to work in connection therewith. The necessity of having the switches and signals co-operate in order to prevent such accidents as are apt to occur from the independent working of the switches from several points was early experienced by the railroads, switches being frequently set for one move and signals for another, causing innumerable accidents.

Railroad Crossings and Junctions. The foundation principle of absolute blocking at crossings and junctions is that all the blocks intersecting at a junction or crossing frog shall be operated as one block. All junctions and crossings should be provided with block signals, interlocked switch signals and derail switches. The latter are opened when the signals with which they are connected are set at "danger," so that a train running past a signal at "danger" will be turned off the track before it can meet a train on another track at the fouling point. Where the track is lower, the derail switch should be placed not less than 500 feet from the fouling point. On a grade descending toward the junction or crossing, the derail should be placed far enough away to give the same degree of safety that it would on a level track at a distance of 500 feet. The block signals, switches and switch signals should be so interlocked that a clear-block signal cannot be given when there is an intervening switch set for a different movement.

Each track must have its own block signals. Where trains proceed in the same direction on different tracks, the signals relating to these tracks are generally placed on the same post or on a bridge spanning the tracks. In the former case the post supports a cantilever bracket. carrying short posts upon which are mounted the signals, the post carrying the main-track signal being higher than the others and nearest the main track. An arrangement of signals for trains in the same direction on different tracks that is simpler is to have the signal governing the main-track movement on a high post by itself and all others on low posts. As all movements from and on side tracks are made at comparatively slow speed, the position of low signals may be readily observed by those interested.

Outdoor Signals. The physical means by which signals are displayed in the block system are of several types, the most common being the semaphore. This is a flat blade or vane of wood or metal, 4 or 5 feet long and 10 to 12 inches wide. One end of this blade

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is supported by a pivot on the signal post, and thus the blade is free to be swung into different positions. In some systems two positions are arranged for, the horizontal and the inclined either upward or downward. In other systems the blade may be set at three positions -horizontal, inclined and vertical-the two latter either upward or downward. These positions are visible only in the daytime, and, for nights, light are displayed automatically, which, by their differing colors, indicate the position of the semaphore blade. This is accomplished by a curved spectacle-like extension at the pivot end of the blade holding two (or three) "roundels" or discs of colored glass before a lamp on the signal post, corresponding to the two (or three) possible positions of the semaphore.

In all systems in which the semaphore is employed, a horizontal position means that the block is occupied by a train and is equivalent to the order, "stop." The roundel which in that position covers the lamp is red. In the two-position semaphore, the "block clear" signal is made by dropping the outer end of the blade to an angle of about 70 degrees below the horizontal. The roundel which corresponds to this position is, in most cases, white; but the larger railroads are, one by one, changing all "white" or "proceed" signals to green. In the three-position semaphore the horizontal position of the blade means "stop." When the blade is dropped to an angle of 45 degrees, or, in some systems, raised to an angle of 45 degrees, the signal means "proceed with caution,» and its equivalent light at night is generally green, though by the new color plan it is a brownish-yellow. In the third position the blade is vertical- either upward or downward

signifying “block clear" or "proceed"; and in the older scheme the light at night is white and by the new color plan it is green. The doing away of the white light makes for a larger degree of safety, for it was not an unusual accident that the roundel should be broken and the light show white when it should have been red. Under the new color plan a white light will mean a broken roundel and hence "danger," and will require a stop and examination as to the position of the semaphore blade.

In the more highly organized signal systems two sets of signals are used, the "home" signal and the "distant" signal; the former name relating to the signal at the entrance to the block, the latter to a signal which indicates the position of the signal at the entrance to the next block. Both are usually on the same post. As a means of distinguishing them, the outer end of the semaphore blade is deeply notched, like a fish tail. It is generally set lower on the signal post. Or, in some systems the "distant" signal is shorter and narrower. The "distant" signal has only two positions - horizontal, meaning "caution" (when approaching entrance to the next block), and inclined, meaning that the next block also is clear. The colors of the lights displayed at night correspond to the "caution" and "clear" colors used on the system. The "distant" signal, therefore, is only a preliminary indication of the signal at the next block and is merely designed to facilitate the operation of a fast train service.

Another signal system which has a large vogue uses the "disc" or "banjo" device instead of the semaphore. The disc revolves on a pivot, so as to show the full face when at danger and only the edge of the disc when the block is clear. A lantern mounted on top of the disc carries two roundels of different colors, showing red when the disc presents a full face and white (or green) when the edge indicates a clear block. A modification of the disc system is arranged so that the disc always presents a full face, with an opening in the centre. Across this opening are thrown colored roundels, red, green or yellow, as the case may be. This system is the one in use on nearly all subway and tunnel traffic and on many electric roads. On the last mentioned the Hall Automatic system, an electric block system, is in extensive use. The instrument which displays the signals is contained in a case mounted on an iron or wooden post, the length of which may be varied to suit condrtions. Within the case is mounted a disc of red silk, with a red glass on the opposite end of the arm carrying the disc. By day a white glass in the back of the home signal case is exposed through the glass-covered aperture in front so long as the large red disc is held out of sight. The front of the case being dark, the indication of safety is thus shown by a white disc in the centre of a dark ground. The falling of the large disc before the glass produces the danger signal, by showing red, and at the same time cuts off the strong contrast formed by the pure white light disc in the centre of the dark case. The front opening is larger than the disc proper, so that when the signal is at "danger" there is a band of white shown around the disc. At night the clear signal is obtained direct from a lamp, showing a white light through the small opening in the top of the case. On the signal assuming the danger position, a ruby-glass lens is carried up before the small opening by gravity, giving a red light. When desired, the "clear" night indication may be green instead of white. In this case the single glass disc is replaced by a double disc, one green, the other red, these being placed at right angles to each other. The distant signal is substantially of the same construction, but the disc is green, and the face of the case may be painted white, to distinguish this signal from the home signal. When at danger the green disc presents a strong contrast to the white surrounding case. At night a green (or orange) lens is used to indicate "caution." At junctions, cross-overs, sidings, etc., a switch instrument is used, which, through its electrical connections, causes the block signal to display "danger" when a switch is not set for the required movement. A visible indicator is placed at main-track switches to warn a trainman desiring to open a switch to let a train out on the main line. This indicator will show "danger" if an approaching train shall have passed a point 1,000 feet or more in advance of the home signal next back of the switch. If the main track is clear and the trainman therefore opens the switch, this operation sets at "danger" the main-track home signal of the block in which the switch is situated.

Besides the three colors mentioned, it is

customary to mark the entrance to a curve in a subway or tunnel with a blue light, so that the running speed may be reduced to prevent undue wear of the rails.

In locating the home signal on single-track lines, care must be taken that the blocks do not overlap. In other words, the home signal governing the entrance to a block for trains running in one direction must be so placed that a train can go to it without encroaching on the block for trains running in the opposite direction. This should be the arrangement of the home signals at every single-track block station, whether there is a passing siding there or not. The rule that a train leaving a block station has the right to proceed quite up to the home signal of the block station in advance should always govern in the locating of home signals. Or, as a general proposition, it may be said of home signals for single-track blocking, that they must be so located that two trains moving in opposite directions toward a common point can be stopped at that point clear of each other, by those signals. Machine blocking undoubtedly offers the safest method of single-track blocking, and is in considerable use on electric roads. The principle involved is a modification of the English staff system. Staff or tablet machines are used, one being placed at each end of each block, and these machines are so constructed that the withdrawal of a staff or tablet from one machine automatically locks the apparatus so that a second staff or tablet cannot be obtained at either end of the block until the first one is restored to one or other of the two machines of the block. The advantages of the electric train staff are (1) that a train can be sent forward from either end of the section without awaiting the arrival of the staff from the other end, a frequent source of delay under the old method, and (2) that greater security in working is obtained on account of the impossibility of withdrawing a staff from the machine at either end of the section without the concurrence of the signalman at the other end. A machine is placed at each end of the block, the two machines being connected by an electric circuit.

The rapid development of railway traffic has created a demand for the power signal in block signaling and interlocking. The power signal has also been developed to meet the increasing requirements and is an important factor in the safe and economical operation of railway trains. In blocking or spacing trains, the power signal in connection with

a

track circuit automatically indicates the presence of a train in the block, a misplaced switch or a broken rail; in routing or switching trains, in connection with electric interlocking, the power signal under control of a leverman, and track circuit also when desired, may be located at any distance from the interlocking tower so that the limits of the interlocking plant are unrestricted; and as a distant signal, in connection with a mechanical interlocking plant, the power signal may be located at a sufficient distance from the home signal to afford proper braking distance for high speed trains.

Since the invention of the closed-track circuit in 1872, the power signal has passed through successive stages of development: