The system is flexible and can be easily ar ranged 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 extend ing 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 Movements in the reverse direction of traffic (or movements) are sometimes necessary and these 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 op erate the switches and signals from a central point is known as an interlocking machine. The fundamental principle of the design of the in terlocking 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 there with. The necessity of having the switches and signals co-operate in order to prevent such accidents as are apt to occur from the inde pendent 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 Manger,* so that a train running past a signal at Manger* 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 sig nals, switches and switch signals should be so interlocked that a clear-block signal cannot he 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 for mer 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 is supported by a pivot on the signal post, and thus the blade is free to be swung into differ ent positions. In some systems two positions are arranged for, the horizontal and the in clined—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 po sitions 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.