Communication-Based Train Control (CBTC) is a modern way for the MTA and subway central operators to know exactly where a train is at any given moment using radio signals, fiber optic lines and advanced software. Currently, the L Line is the only one in the entire subway system to have it installed.
In this video from the Regional Plan Association, learn all about the CBTC technology and how it works.
Presently, New York City transit uses a technology whose origins are more than 100 years old called fixed-block signals, which relays information about where trains are more primitively. Using electrical currents, trains are run in 1,000-foot sections, on average, and no two trains are allowed in any one block. Because it is less precise, trains have to run further apart, as well as run more slowly and be delayed more often, to prevent collisions. CBTC technology allow trains to run faster and closer together, effectively expanding the capacity of the system at a time when ridership is growing every year and crowds on platforms and in trains are common.
Check out the accompanying website for more info.