21st-Centuary Technology

Block signaling will finally begin to yield to the computer age in July, when the New York subway inaugurates a new signaling system for its L line. Two-way radios link each train to way stations along the route and from these to a centralized command center.

To capture the position and speed of each train, a laser on the train's undercarriage shines a beam directly onto a running rail. The reflected light registers in a small infrared camera mounted on the train beside the laser.

A custom-designed 2.4-gigahertz spread-spectrum radio system relays this information to the command center's computers, which integrate it with information on the length of the train, the location of stations, and the curves and changes of grade along the route. The computers calculate when the train should apply its brakes to maintain a safe distance from the train in front of it or to make a station stop.

Control is so precise that trains can barrel down the tunnel 60 seconds apart, or even less. The computers will thus speed more trains on their way and will one day even tell people in the stations when the next train is due to arrive.