CBI

Computer-based interlocking is a railway signalling system in which a computer enforces the rules that prevent conflicting train movements. Instead of mechanical levers or relay logic, a PLC or dedicated industrial computer reads signals from track circuits, switch position sensors, and operator input, then decides which signal aspects to display and which points (switches) to move. The computer will not permit a signal to clear for a train if the route ahead is occupied, if points are set incorrectly, or if conflicting movements would result.

CBI systems replaced electromechanical interlocking cabins that used banks of relays and latching circuits to enforce safety logic. A traditional relay frame might occupy a substantial building; a CBI installation fits in a cabinet. The transition began in the 1980s and 1990s, with systems now standard in new installations and major resignallings across Europe, North America, and Australia. Most modern metro systems, commuter railways, and busy regional routes run on CBI.

The computer continuously scans inputs (track occupancy detectors, switch position microswitches, push buttons from the signaller) at intervals typically measured in tens of milliseconds. It then executes logic code stored in memory that mirrors the old interlocking rules: if this route is requested and that section is clear and those points are in the right position, then that signal may be cleared. The outputs drive signal lamp circuits, point motor relays, and level crossing barriers. Failsafe design is essential. Most CBI systems use redundant processing, watchdog timers, and coded data transmission so that loss of communication or a processor fault results in signals reverting to danger rather than danger being overlooked.

A key advantage is flexibility. The signalling logic can be reconfigured in software when a layout changes, without rewiring relay racks. Remote operation of signals and points becomes practical because the computer handles communications and safety checking; a signaller at a panel miles away can control a junction or yard. CBI also enables automatic train protection schemes and integration with traffic management systems that optimize routing and timetable adherence.

The term CBI is now somewhat dated in technical circles. Suppliers and operators increasingly use more specific names such as ETCS Level 1 (European Train Control System), CBTC (Communications Based Train Control) for driverless metro systems, or proprietary names like Alstom's SMARTLOCK or Siemens S700K. However, CBI remains in use as a broad category, especially in heritage contexts or when distinguishing older computer systems from microprocessor-based signalling.