Railway signalling and CBTC is the system that tells each train when it is safe to move. CBTC stands for communication-based train control. Without signalling, two trains could end up on the same stretch of track.
Traditional signalling divides the track into stretches called blocks. A signal at the start of each block shows red if a train is already inside it, and the next train must wait. So only one train is ever in a block, and the gap between two trains is at least one block long.
Stations and junctions need a second safeguard. There, tracks join and cross through movable sections of rail called points. Interlocking is the part of the signalling system that makes sure the signals and the points never contradict each other. It will not show green for a route if the points are set the wrong way, or if another train has been given a route that crosses it. Many Indian stations are moving from interlocking worked by mechanical levers to electronic interlocking, in which a computer makes this check.
CBTC removes the block limit. Each train reports its exact position by radio, all the time, to computers along the line. These computers know where every train is and how fast it is going, and they send each train a limit on how far it may move. So trains can safely run closer together, and more trains can run on the same line. On a busy metro line this matters, because building a second line costs far more than upgrading the signals.
Imagine a metro operator whose trains run every four minutes because of the block length. With CBTC, the operator might shorten the gap to about two minutes. The same tracks then carry nearly twice as many passengers. These numbers are only an illustration, because real gaps depend on the line.
Signalling decides when a train may move. If the driver ignores a red signal, automatic train protection brakes the train. Signalling needs the position of every train, and it passes data about its own equipment to condition monitoring. Signalling is a safety system, so it must pass safety certification. Railways buy it through tenders and use it for decades, as described under railway technology.