Radio-based railway signalling that tracks every train continuously, letting trains run closer together than fixed track blocks allow.
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Communications-based train control is signalling that talks. Older systems cut the track into fixed blocks and allow only one train per block; because a block must be at least as long as a worst-case stopping distance, trains keep a gap sized for a train that is not there. CBTC, defined by the standard IEEE 1474.1, has each train report its own position by continuous two-way radio, so the system can compute a moving block — a safety envelope that travels with the train and shrinks as it slows.
The payoff is capacity inside tunnels a city already owns: more trains per hour without digging. Toronto swapped 1950s fixed-block signalling on Line 1 for moving-block CBTC and says peak service rose from about 25 trains an hour to about 30. CBTC is also the precondition for higher grades of automation — but only the precondition. Melbourne's Metro Tunnel opened on CBTC in 2025 with a driver still in the cab. The radio decides how close trains may run, not who is on board.
Retrofits are the hard part. Fitting new signalling to a railway that has to keep running is slow and expensive, and the supplier picked today shapes the network for decades. Toronto finished seven years past its original target. On Boston's Green Line, which still has nothing enforcing its speed limits, the NTSB asked for collision avoidance in 2009 and automatic braking is not due before 2028.
Added 2026-08-01 · Part of the growing Smart City Glossary