SSI is a computerised interlocking system which emphasises creating safe and reliable interlocking. Communication between the trackside equipment and the central interlocking is run over a pair of wires forming a ‘datalink’ for reliability, another separate datalink is run in the opposite direction. These datalinks can be run for considerable distances if required. Commands from the central interlocking are received and validated by the target trackside functional modules prior to being acted upon. The trackside functional modules then report back both equipment and diagnostic data to the central interlocking to enable the correct information to be given to the signaller and for fault diagnosis.
