Computer-based interlocking

Computer Based Interlocking is the generic term for railway signal interlocking implemented with computers rather than older technologies such as relays or mechanics.
General.
CBIs are mostly implemented in two parts; a section that implements the safety and failsafe requirements, and a second section that implements "non-vital" controls and indications.
Brands.
Different manufacturers have their own brands of CBI such as
Interface between different brands.
When interfacing different brands of CBI equipment, it may be necessary to use relays of each regime, which are then hardwired from one to the other. This happens in the middle of the Channel Tunnel where French and British signalling equipment meet.
Competition Rules.
Since only some of the major signalling organisations make CBI equipment, it was agreed that to prevent other players being left out in the cold, CBI equipment would be made available to those players at cost.