Safe operation of freight and passenger trains requires a system of signaling. To inform the locomotive and train crew of the position of other trains in relation to their own, sig¬nals installed at frequent intervals give indications which are visible both by day and by night. Wayside signals in¬stalled along railroad tracks are called fixed signals.
We know that the semaphore used to be the most common type of the signal. The relative position of the semaphore arm constituted the signal.
To indicate "stop" a horizontal arm was used. "Proceed" was indicated by a vertical arm. To give restrictive (i.e. cau¬tionary) indications the arm was inclined up or down.
Colored lights give the indications at night. The sema¬phore mechanism is equipped with lenses illuminated by a lamp, so that a red light shows when the semaphore is in the "stop" position, a green light — when the semaphore is in the "proceed" position, and a yellow light — when the semaphore is in the restrictive position. The color-light signal some¬times used is known to have semaphore arm and give both day and night indications be means of red, green and yellow lights. We know some signals to be operated by hand, others to be automatic.
Locomotives on some railroads are known to be equipped with apparatus located in the cab, which gives a continuous indication to the engineman identical with that shown by wayside signals.
By cab signals the engine crew is supposed to be always informed of conditions ahead regardless of the weather that affects the man's ability to see wayside signals. Locomotive cab signals are equipped to give audible warnings whenever the aspect changes to one more restrictive. A protective de¬vice is installed on some railroads to apply the brakes auto¬matically and bring a train to a stop if, for any reason, a "stop" signal should be passed. It is called automatic train control. The first signals installed are known to have been hand-operated, usually by station employees.