In 2020, Solid State Interlocking (SSI) will be 35 years old. It was one of the first microprocessor-based all electronic signalling interlockings and is considered by many to be a world beating railway technology.
SSI was developed in the 1980s under a tripartite agreement between the British Railways Research Department and the two major UK signalling suppliers, Westinghouse Signals (later acquired by Invensys and now Siemens) and GEC (later absorbed by Alstom).
The very first use of SSI was at Dingwall in 1984 as part of the ‘Radio Electronic Token Block’ (RETB) signalling on the ‘Far North’ and Kyle of Lochalsh lines in Scotland, followed in August 1985 with the first conventional SSI installed at Leamington Spa. This provided signalling control of colour light signals and point machines, initially from an entrance-exit panel.
SSI is installed at over 450 sites in Great Britain, which constitute approximately 45 per cent of UK interlockings. With older parts of the signalling system first requiring renewal, it is likely to be required for many years to come. Another 1,000 systems are in use throughout the world abroad, including South Africa, Indonesia, Hong Kong and Australia.
Despite being over 30 years old (the typical life for such systems in other industries is ten years), SSI has been a huge success and was generally accepted as being ahead of other such systems around the world. It inspired the next-generation systems developed by the two manufacturers, Trackguard Westlock from Siemens and Alstom’s Smartlock 400, which have the capability for backwards compatibility with the classic SSI application data and with trackside equipment.
Given that SSI was developed over 30 years ago, and that many other microprocessor-based electronic systems are obsolete after a few years, the core SSI system is remarkably still fit for purpose, although, not surprisingly components such as the Technicians Terminal are now obsolete, both in hardware terms and in functionality. The original terminal only had an expected life of 10 years at the most, and its green-on-black monochrome display and command input operation is a world away from modern ‘mouse driven’ interactive terminals which we all use every day.
Park Signalling, now a Unipart Rail company, was formed in the year 2000 by key staff from the Manchester office of Alstom Signalling which had helped to develop SSI. They named the company ‘Park’ after ‘Trafford Park’ where they were originally located, although they have since relocated to Stockport.
The twenty full time staff have many years’ experience of SSI-based and other signalling systems, which is supplemented with a number of associates and specialists who are called upon to provide expertise when required. Park Signalling operates in three main markets: extending useful life and enhancing performance, system upgrades and enhancements, and designing bespoke products and systems.
With many of the Park Signalling engineers originally responsible for the design and development of SSI for GEC/ALSTOM, they are ideally placed to continue to design, develop and manufacture ‘smart tools’ that support and improve SSI. Knowing the design of SSI from first principles, combined with extensive specialist equipment resources, allows them to identify and diagnose the root cause of reliability and performance issues.
The MT04S Technician Terminal provides a smart, powerful tool to monitor and analyse SSI data links. It has been developed using standard commercial off-the-shelf (COTS) components with technologies chosen for their longevity. It has now received full and final product acceptance from Network Rail.
MT stands for Maintenance Terminal and, with it being the fourth maintenance tool developed by Park Signalling, the SSI Technicians Terminal is called MT04S. The electrical interfaces are directly compatible with the original terminal, and it replicates the feel and functionality of all original and existing equipment, but with a modern graphical user interface.
The terminal has been developed to be more intuitive, easier to use and to display information more clearly. USB pen drives are provided for event logging and event data recovery in a controlled manner. It can connect and control up to six SSI systems.
In detail, the Technicians Terminal provides the means of applying and removing technician controls, such as to control the stop/start interlockings, track circuit occupancy, route barring, aspect/points disconnection/disablement and temporary approach control. The terminal can display datalink telegram contents and panel requests. It provides additional functionality over the original terminal, such as built in event analysis and the labelling of trackside functional module input/outputs when monitoring. This is far easier to use than with the original terminal.
A user group is planned by Park Signalling for the product, to increase and add to the existing functionality, and to meet the requirements of the maintenance areas across the country. Already, the company has received requests for remote access and a system to only allow access to users with the appropriate competency, and to log all activity by user. With a modern Technician Terminal now available, these are the sort of enhancements that can be developed jointly with the industry and rolled out for the benefit of everyone.
SSI link monitoring
Having fault-tolerant systems is great, but if operators can’t identify they have a problem and do something about it, they are just putting off the inevitable failure and not avoiding it. One aspect of SSI that heavily impacts on signalling availability is that the data links are remarkably tolerant of problems and performance issues. This can be masked by the SSI diagnostic processor, which only reports a complete failure. So, if a data link cable is degrading slowly, say from a faulty cable joint allowing water ingress, the corrupted data telegrams may not be noticed until the system fails.
Fortunately, Park Signalling has tools to address these issues that work with SSI. There is a Remote Missing Telegram Detector system that monitors and reports missing and corrupt telegrams, providing not only a continuous count of missing telegrams, but also a count for individual telegram addresses, which is particularly helpful in diagnosing the location of specific data link faults.
A second tool monitors the baseband links and (potentially) eliminates the need to carry out regular checking, providing faulting assistance when data link problems occur and allowing faster restoration of the link.
Having the ability to manage assets via smart, powerful tools, analyse data and predict failures is the sort of facility which is expected and standard on any modern computer-based control system, but now, thanks to Park Signalling, it is now available for good old SSI.