Last October Network Rail announced that they will be phasing out miles, chains and yards on the British Rail network in favour of metric units. This has already been done on most, if not all Britain’s metro and tram systems while the dimensions of railway vehicles have been measured in metric units since the 1970s. Why this change of heart?

(Article contributed by Martin Vlietstra)

Railway inter-operability has plagued railway authorities for many years. In the early years each railway company had its own standards. Although many countries managed to harmonize standards (to a degree) within their own borders, international rail travel presented many incompatibilities – rail gauges, loading gauges, electrical power supplies to mention but a few. Today Spain is in the process of building new high speed railway lines to Standard Gauge (1435 mm) rather than the older Iberian Gauge of six Castilian feet (1672 mm) while the early models of Eurostar had to contend with 25 kV AC overhead lines in France, 1500V DC overhead lines in Belgium and 750 V DC third rail supplies in the United Kingdom. This makes metric conversion look like a doddle!”

The advent of high-speed trains such as the TGV brought a new set of problems. While the problems of loading gauge and voltage changes could be overcome, signalling became a major problem. In short, at high speed, train drivers could not read line-side signals. This meant transmitting signals from line-side equipment to the driver’s cab by radio. More technology meant more standards. Higher technology meant longer design and testing processes. Part of the EU’s support for a pan-European rail network was the adoption of a common signalling standard. EU directives, published in 1991 and 1993 required the use of ETCS (European Train Control System). ETCS is an essential component of the ERTMS (European Rail Traffic Management System), a system that is being introduced on many European railways.

The principle behind ETCS is that information that might be displayed on line-side signals is transmitted to the driver via Eurobalaises – boxes that are typically 30 mm high and mounted in the centre of the sleepers. These boxes are linked to a central computer and transmit information to and from trains as they pass over the balaise. The driver has a standardised box in his cab which displays the information. ETCS cannot be introduced overnight. The specification has four levels ranging from Level 0 where the ETCS equipment merely supplements existing line-side equipment to Level 3 where ETCS takes full control of the train. Given its international heritage, it is unsurprising that ETCS and ERTMS use metric units.

In 2006 the UK took its first steps in introducing ERTMS. The 218 km Cambrian line (which runs from Shrewsbury across mid-Wales to the coast) was in need of resignalling. Since the line was isolated from the rest of the UK network, it was decided to use Level 2 ERTMS on this line to gain experience in the installation and use of the system. In Level 2 ERTMS, line-side signals are present but are theoretic ally redundant as all signals are passed through the Eurobalaises to the driver. As part of the Cambrian resignalling process, all mileposts were replaced by kilometre markers and the rule book rewritten in km/h rather than mph. Network Rail learnt many lessons from this exercise, not least that overcoming procedures that are unique to Britain require solutions that are likewise unique to Britain.

The statement by Network Rail said that the Rail Safety and Standards Board are assessing the impact of the changeover on safety though at the moment they have no timetable. The current plan is to introduce ERTMS on a phased basis – the Great Western line between 2014 and 2018, the East Coast Main Line between 2018 and 2020, the Midland Main Line between 2020 and 2023 and the West Coast Main Line in the late 2020s. It remains to be seen whether the metric change-over will be done well in advance of the ERTMS installation or whether it will be done as a “just-in-time bodge”.

Links

http://www.railnews.co.uk/news/2013/10/01-end-of-the-line-for.html

http://www.uic.org/cdrom/2012/10_ERTMS-Conference2012/docs/C4-Morgan-LeppardV3.pdf

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