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Block System. The term block as used above applies to a certain length of track each end of which is protected by means of a distance and home signal. The length of a block varies through wide limits depending upon the nature of the country, amount of traffic, and speed of trains. The heavier the traffic, the more trains there are to be run, so it is desirable to run the trains as close together as possible. Hence, the blocks should be as short as safety will permit. On the other hand, as the speed of the train increases, the time required to pass over a given distance is diminished, hence the length of a block may be increased. The length of the block differs for single-, double-, and four-track roads. Ordinarily the blocks are from ten to twelve miles long. There are a number of different kinds of block systems named as follows, according to the way in which they are operated: the staff, controlled manual, automatic, and telegraph systems. All of these systems are similar in their principle of operation, differing only in the means used in securing the desired results. For instance, the controlled manual is operated by a tower man but the mechanism is partly automatic so that he cannot throw his signals until released by mechanism at the other end of the block which electrically locks his signals.

The working of the lock and block system between two stations A and B, Fig. 121, is as follows: When a train approaches A, the operator pulls his signal to clear, provided there is no other train in the block. As the train passes the signal and over a short section of insulated track, the wheels short circuit the track which carries an electric current. This action operates electrical apparatus which permits the semaphore arm to go to the danger position by force of gravity. After the operator has cleared the signal, an electric locking machine works in such a way that the signal cannot again be cleared until the train has passed over another section of insulated track as it passes out of the block at the station B. When the train passes this second section of track and short circuits the track, an electric current is automatically sent back through line wires to Aand unlocks the machine, giving the operator at A permission again to clear his signal permitting another train to enter the block.

The above description of the lock and block or controlled manual system will make clear the following established principles of interlocking:

  1. Each home signal, lever in that position which corresponds to the clear signal must lock the operating levers of all switches and switch locks which, by being moved during the passage of a train running according to that signal, might either throw it from the track, divert it from its intended course, or allow another train moving in either direction to come into collision with it.

  2. Each lever so locked must in one of its two positions lock the original home signal in its danger position, that position of the lever being taken which gives a position of switch or switch lock contrary to the route implied by the home signal when clear.

  3. Each home signal should be so interlocked with the lever of its distance signal that it will be impossible to clear the distance signal until the home signal is clear.

  4. Switch and lock levers should be so interlocked that crossings of continuous tracks cannot occur where such crossings are dependent upon the mutual position of switches.

  5. Switch levers and other locking levers should be so interlocked that the lever operating a switch cannot be moved while that switch is locked.

Levers at one signal station are locked from the station in advance. Thus, the signal A, Fig. 121, cannot be put to clear until freed by the operator at B. B cannot be cleared until freed by C, etc. Levers and signals may be operated by hand, pneumatic, or electric power, the last two either automatically or by an operator.

Hall Signal. Disk signals are also used for block signaling and are usually automatic. The Hall signal, illustrated in Fig. 122, is an example of this kind. It consists of a glass case A containing electric apparatus operated by a current controlled by the passage of a train. When the block is closed, a red disk fills the opening B by day, and a red light shows at C by night. A clear signal is indicated by a clear opening at B by day and a white light at C by night.

When a single track is to be operated by block signals, it is customary to put two semaphores on one pole, as shown in Fig. 123. The arm extending to the right as seen from an approaching train is the one controlling the movement of that train.

Dwarf Signals. These are in all respects similar to the regular semaphore differing only in their size. They are usually short arms painted red, standing from two to four feet from the ground, and are similar to the home signal. They are used only to govern movement for trains on secondary tracks or movements against the cunent of traffic on main tracks when such reverse movement becomes necessary, and where necessary in yards. They are especially used for governing the movement of trains in backing out of train sheds at terminals.

Absolute and Permissive Block Signaling. Block signaling should always be absolute, that is, when the home signal is at danger no trains should be allowed to pass. It should never be cleared until the whole block in advance is emptied; that is, the signal at B, Fig. 121, should never be set to clear until the last preceding train has passed the home signal at C.

Permissive signaling introduces a time element into the system and is practiced by many roads. Thus, when a certain time, usually from 5 to 10 minutes, has elapsed after a train has passed a home signal, a following train is allowed to proceed though the signal still remains at danger. The following train is notified of the occupancy of the block by the preceding train by the display of a cautionary signal, usually a green flag or light from the tower at the signal so passed. It is a dangerous system and one subversive of good discipline and safety.

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