Behind 19th-Century Technology

When the route affected by the 25 January blaze opened for passenger service in 1932, it was the envy of the other privately owned subway lines that ferried people back and forth to work. It was the first to have its entire route governed by automatic wayside electric light signals. Along the tunnels, red, yellow, and green traffic signals [photo] similar to the ones guiding automobile and trolley traffic on the streets above told a train operator whether it was safe to proceed.

To keep track of each train and so prevent collisions, the signal system marked off electrically isolated sections, or blocks, along one of the running rails. Each block is connected to a signal by circuits, relays, and wires to form a low-voltage ac circuit. When Train A is on that portion of track, the circuit is broken, and this information is relayed to a signal room, such as the one destroyed in the fire. The room automatically directs the signal a few sections behind Train A to alert the operator of Train B to slow down or stop until the first train has completely cleared its section of track.

Photo: New York Transit Museum