Computer Based Interlocking is the general term for a railway signalling interlocking implemented with computers rather than the older technologies such as relays or mechanics

Mechanics

Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....

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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

• SSI
  Solid State Interlocking
  Solid State Interlocking is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail's Research Division, GEC-General Signal and Westinghouse Signals Ltd in the UK.- Interlocking hardware :...
  
   (Solid State Interlocking) - BR
  British Rail
  British Railways , which from 1965 traded as British Rail, was the operator of most of the rail transport in Great Britain between 1948 and 1997. It was formed from the nationalisation of the "Big Four" British railway companies and lasted until the gradual privatisation of British Rail, in stages...
  
  , Invensys
  Invensys
  Invensys plc is a global engineering and information technology company headquartered in London, United Kingdom. It was formed in 1999 through the merger of BTR plc and Siebe plc. It has offices in more than 50 countries and its products are sold in around 180 countries.Invensys is organised into...
  
  , GEC-General Signal

• Mircrolok II

• Smartlock

Competition Rules

Since only some of the major signalling organisations make CBI equiment, 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.