Metrication of the rail network

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”.


5 thoughts on “Metrication of the rail network”

  1. Excellent article Martin. On a point about the Spanish rail gauge, the old Spanish gauge was indeed six Castilian feet (1672 mm). Portugal’s old gauge was five Portuguese feet (1664 mm). In 1955, Spain and Portugal agreed to split the difference, resulting in today’s Iberian gauge of 1668 mm. The new 320 km/h high speed lines connecting with France are to Standard gauge (1435 mm), and there are long range plans to convert the whole of Spain’s rails (and presumably Portugal’s too?) to Standard gauge. Considering Spain’s chronic economic situation, I can’t see this happening anytime soon, especially as much new rolling stock has dual gauge wheel sets, with gauge changing facilities at most breaks of gauge, while some stretches of track are built to dual gauge.

    Going back to the thrust of the article, it has always seemed incongruous to me to see distances on bridges and such like measured in chains. I believe that the UK has formally signed up to ERTMS, which will mandate the use of metric units throughout Britain’s railways. Not before time.

    Finally, Martin notes that tram systems use metric units. And that means that on such systems, where they share roads and other rights of way with the general public, drivers are then faced with incompatible measurement units – metric in the cab and imperial on road signs. One more reason to complete conversion of Britain’s road signs.


  2. Next step would be to persuade motoring journalists not to mix Imperial and metric in the same sentence.
    “1.5-litre, 106bhp petrol unit of no particular virtue. You might think 106bhp is plenty for a little car, but it’s a heavy one at 1,150kg.”
    “Call yourselves professional journalists? I’ve known smarter washerwomen.”


  3. Just 15 months later (24 Jan 2015) and “The Mail on Sunday” has caught up and published this article .
    Now, whether or not the EU has had any hand in this I do not know for sure, but having been pre dated by 15 months my guess would be the EU has just ‘rubber stamped’ a decision by Network Rail, thus removing a previous opt out that is no longer required.
    Two points come to mind. Firstly, whatever truth there may be in the article, it is way off the mark and I feel there must surely be some limit as to the degree of ‘economy of truth’ any media can get away with. The second is that this shows how biased the media still is to a rational decision by a company to do the right thing and complete the obvious eventual changeover.


  4. The switch to metric will take place over the next two decades??????

    These people are mad, I tell you. Why not just two years, maybe (if that)?

    This is pure Chinese torture. But at least the request from DfT for an opt-out was declined!


  5. It is interesting that the Mail article mentions a measurement unit called the chain. This was introduced in 1620 by Edmund Gunter (1581-1626), an English mathematician and clergyman. The chain was 22 yards long and had 100 links of 7.92 inches. It reconciled two seemingly incompatible systems: the traditional land measurement system based on the number 4 and the, then innovative, decimal system, thereby simplifying calculation. Thus, an acre is 10 square chains or 10^5 square links or 4840 square yards.

    The Gunter chain was popular with the engineers constructing Britain’s railways in the nineteenth century, both for surveying the parcels of land required and for setting out the works. For example, the original gradients on the GWR between Paddington and Reading were: 1/1760 (1 ft/80 ch), 1/1210 (1 ft/55 ch), 1/1320 (1 ft/60 ch), 1/1650 (1 ft/75 ch), 1/880 (1 ft/40 ch) and 1/770 (1 ft/35 ch). Today, structures on much of the national rail network are marked with the distance in miles and chains from the start of the line.

    However, strictly speaking, the chain is not an Imperial measurement – it is not mentioned in the Act of 1824 which created the Imperial system. This Act does however confirm the medieval land area measures of the rood (acre/4) and the square perch (rood/40). History, of course repeated itself in the 1970s, when engineers adopted more rational units of measurement while legislators preferred to remain in the past, as they still do with a few notable exceptions.

    In the eighteen century, Jesse Ramsden, a scientific instrument maker, introduced a chain of 100 links of 1 foot. Popular in North America, it never caught on in Britain, although I remember using one during an engineering surveying course in 1962.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: