It's a great deal more complicated than three lights.

"[Communications-based train control] requires a set of equipment on trains and at trackside to gather and transmit data to central computers, and to receive instructions back for train operation. ATC beacons and axle counters sit at track level to detect the passage of a train with the axle counters backing up the beacons. The track-level beacons transmit signals to onboard controllers installed on each train. Onboard roof antennas (Data Communications System antennas) transmit information such as speed and location from the controllers to Trackside Radio Equipment which relays the data to central computers and to the TTC's Transit Control Centre. The central computers send speed and braking instructions back to the train, whereby the central computers are effectively directing each train. With computers operating the train, trains can operate safely on closer headways than with the fixed block system. CBTC allows more frequent service and increased line capacity.[3]"

https://en.wikipedia.org/wiki/Signalling_of_the_Toronto_subway