Fixed Signals. Fixed signals consist in the use of posts or towers fixed at definite places and intervals having attached to them a system of rods, levers, and bell cranks to properly operate the arms or semaphores. The target is one form of fixed signal.
Targets are used to indicate, by form or color or both, the position of a switch. A target usually consists of two plates of thin metal at right angles to each other attached to the switch staff. The setting of the switch from the main line to a siding, for example, turns the staff through a quarter revolution thus exposing one or the other of the disks to view along the track. The disks or targets are usually painted red and white, respectively. When the red signal is exposed, the switch is set to lead off to the siding. When the white one is exposed, the switch is closed and the main line is clear. At night, a red and green or red and white light shows in place of the target.
The semaphore may now be considered as the standard method of controlling the movement of trains. It consists of an arm A, Fig. 117, pivoted at one end and fastened to the top of a post. When in the horizontal position, it indicates danger. When dropped to a position of 65 or 70 degrees below the horizontal, as in Fig. 118, it indicates safety.
At night, the semaphore is replaced by a light. There are two systems of light signals; one is to use a red light for danger, a green light for safety, and a yellow light for caution. The other is to use red for danger, white for safety, and green for caution. The method of operation is to have a lantern B, Fig. 118, attached to the left-hand side of the signal post in such a position that when the semaphore arm is in the horizontal position, the spectacle glass C will intervene between the approaching engine and the lantern as in Fig. 117. This spectacle glass is red. Where green is to be shown with a semaphore in the position shown in Fig. 118, the spectacle frame is double, aa in Fig. 119, the upper glass being red and the lower green.
Semaphore arms are of two shapes, square at the ends as in Figs. 117, 118, and 119, and with a notched end, as in Fig. 120. The square ended semaphore is used for what is known as the home and advanced signals, and the notched end for distance signals. Semaphores are set so as to be pivoted at the left-hand end as viewed from an approaching train. The arm itself extends out to the right.
The use of home, distance, and advanced signals is as follows: The railroad is divided into blocks at each end of which a home signal is located. When the home signal is in a horizontal position or danger position, it signifies that the track between it and the next one in advance is obstructed and that the train must stop at that point.
The distance signal is placed at a considerable distance in front of the home signal, usually from 1,200 to 2,000 feet, and serves to notify the engineer of the position of the home signal. Thus, if when he passes a distance signal, the engineer sees it to be in a horizontal position, he knows that the home signal is in the danger position also and that he must be prepared to stop at that point unless it be dropped to safety in the meantime. The distance signal should show the cautionary light signal at night.
The advanced signal is used as a supplementary home signal. It is frequently desirable, especially at stations, to permit a train to pass a home signal at danger in order that it may make a station stop and remain there until the line is clear. An arrangement of block signals is shown in Fig. 121. There are three home signals A, B, and C on the west bound track, the distance between them being the length of the block. This distance may vary from 1,000 feet to several miles. D, E, and F are the corresponding home signals for the east bound track. The distance signals G, H, I, and K protect the home signals B, C, E, and F; L is the advanced signal at the station M for the home signal B. Thus, a train scheduled to stop at M will be allowed to run past the home signal at B when it is at danger and stop in front of the advanced signal L. When L is lowered to safety, the train can move on.
The signals of the block are usually interlocked, that is, one signal cannot be moved to danger or safety until others have been moved. The signals of two succeeding stations are also interlocked, usually electrically.
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