scope:

General

This document describes the modifications to standard SSI data and interface wiring that are necessary in order to design an SSI replacement for a relay based interlocking, whilst retaining the existing external lineside equipment and cabling.

This method would usually be deployed at sites where the interlocking wiring within the relay room is in a degraded state, but the external wiring and equipment are still serviceable.

Trackside Functional Modules (TFMs) are used to drive output interface relays. The output interface relays control the existing relay room outgoing lineside circuits. Similarly, the existing incoming lineside circuits are used to operate input interface relays. TFM inputs will be taken over contacts of these relays.