Solid State Interlocking

SSI utilises a 2-out-of-3 redundancy architecture, whereby all safety-critical functions are performed in three separate processing lanes and the results voted upon.

Geographic interlocking data, relating to the area of railway under control, is installed using EPROMs contained in plug in memory modules.

A maximum of 63 TFMs may be addressed by one SSI interlocking; in practice the number will be limited by timing issues and the need to allow for future expansion.

The second phase of the Docklands Light Railway in 1987, including the extension to Bank, the re-design of the Poplar / Canary Wharf layout and the reduction of the technical headway to under 2 minutes, saw the expansion of the SSI installation.

Australia is an extensive user of SSI, particularly New South Wales, where it is installed at busy locations such as Hurstville - Oatley, North Sydney, Wyong, Granville, Enfield, Blacktown, Olympic Park and others within the Transport Asset Holding Entity network.

Due to the success of SSI within the UK market, Alstom and WRSL have released products (Smartlock and Westlock respectively) which copy a number of its features.