From 1520 to 3 – metric still rules (mostly)

A first for Metric Views – an article on model railways. John Frewen-Lord takes us to an area that many of us last visited in our childhood.

My last Metric Views article on Russian 1520 mm gauge railways elicited such a deluge of responses (well, all of three actually, other than my own) that I thought I’d continue on this theme, but this time with the focus on model railways. And it is here that we find the use of the metric system extremely well entrenched (albeit with a unique British twist), even extending to the very nomenclature inherent in the different scales and gauges encountered.

Model railways (or perhaps more like toy trains) first became available in quantity in the early years of the 20th century, when German manufacturers such as Märklin and Bing started to produce electric O-scale metal (tin-plate) trains, initially running, unbelievably, on mains voltage (220 V – which must have made derailments quite spectacular). In the USA, Lionel and other manufacturers also started to produce O-gauge (as it is also commonly called) trains soon after.

Now O-scale meant (and still means) a scale of between 1:48 and 1:43 (i.e. any dimension on the real thing is between 43 and 48 times greater than the model), and is sometimes known as 7-mm scale. Anachronistically, this means a scale of 7 mm to the imperial foot (1:43.5). This unique form of nomenclature is popular only in Britain (the USA uses the terminology of ¼ in to the foot, while other countries simply refer to it as the ratio). The track gauge – the distance between the rails – can vary between 30 and 33 mm for 1435 mm Standard gauge, depending on the exact O-scale used, although the US-based National Model Railroad Association (NMRA), which has become a world leader in setting model railway standards, specifies a preferred gauge of 31.75 mm.

Between the two world wars, and especially after the Great Depression, there was a demand for smaller trains, and the scale of HO was introduced – meaning Half-O. Naturally this became known amongst the imperial-measuring British as 3½ mm scale (again to the foot). European manufacturers still predominated, with Märklin, Fleischmann and others producing high quality models to HO scale. (My father was in the RAF, and during his many trips on the Berlin Airlift in the late 1940s managed to bring home a complete Märklin HO system long before Märklin was commercially available in the UK. They were indeed exquisite models.)

HO-scale has a track gauge of 16.5 mm representing Standard gauge at 1435 mm, and this gauge is only ever known as ‘sixteen point five’ everywhere, even among people whom you might expect to be anti-metric – I’ve never heard it expressed in any other way (and I do maintain an interest in model railways). Unlike with O-scale, HO-scale is universally recognised as being to one fixed scale – 1:87, exactly half of O’s 1:43.5 – and this is the scale that manufacturers throughout the world (including the USA) stick to in producing HO-scale models. Except of course in Britain.

During the 1930s, Britain’s largest toy manufacturer, Hornby, also wanted to start producing electric HO trains (they already had a large range of tin-plate O-gauge trains, mostly clockwork). The problem is that British trains in real life have a much smaller loading gauge (the height and width of the trains) compared to their European counterparts (and especially compared to trains in North America), and this meant that HO-scale British trains would be too small for the manufacturing capabilities and motors of the day. Enter the unique 4 mm to the imperial foot scale. This represents a scale of 1:76, and as well as in Britain it was also to be found to a very small extent in Germany. Yet the track gauge was maintained at HO’s 16.5 mm, and so the scale became known as OO – commonly expressed as Double-O. With a stroke of typically British marketing genius, Hornby called their trains – wait for it – Hornby Dublo.

In any hobby or interest, there are usually those who might be described as purists (and there are many in the field of metrication). The model railway world is no exception, and such model railway purists (often disparagingly referred to as rivet-counters) decried the mixing of HO track with OO trains. So they invented their own solution – which was not to reduce the trains to a scale of 1:87, but to increase the track gauge to OO’s scale of 1:76. This produces a preferred track gauge of 18.2 mm (which is really the conversion of 4 mm to the foot). These purists then set up their own society to promote the interest in exact 4-mm scale modelling – which they called the EM Gauge Society. EM is derived from Eighteen Millimetres – again metric nomenclature in what is a uniquely British (and otherwise imperial-oriented) aspect of model railways (not sure what happened to the missing 0.2 mm.)

In the post war years, houses got ever smaller, and consequently there was an increasing demand for something more compact than HO. The solution was N-scale – generally 1:160 (except – where else? – in Britain, where it is 1:148). So where did the name ‘N’ come from? This is another bit of metric nomenclature inherent in model railways.  The track gauge for 1435 mm Standard gauge at 1:160 works out at 9 mm – or Nine millimetres. Even the British, using a scale of 1:148, again because British rail vehicles were too small otherwise to accommodate available motors, and mirroring what they did with OO, stuck with 9 mm track gauge. (The scale of 1:148 by the way is also another British anachronism resulting from the use of describing N-scale as 2 mm to the foot.)

Once we get below N-scale, there’s not too much available.  Märklin have an offering called Z-scale, at 1:220 (Z being chosen, not because of any metric connotations, but because it was considered that nothing smaller could possibly be made). Being German it is to all-metric standards of course, and not even the British have dared to try some variation of this. Z-scale track gauge is a very small 6.5 mm. Again, being German, these trains are incredibly well engineered – a Märklin Z-scale train travelled non-stop for over 1200 hours and covered a true 720 km, about 158 000 km scale distance, and putting itself into the Guinness Book of World Records.

The last word however has to go to the Japanese. Known as T-gauge, they have made model trains to an infinitesimally small 1:450 scale. It is called T-gauge for the same reason as 1:160 is called N-gauge – the gauge of the track, which in this case is a tiny Three millimetres. Models are appearing that represent not only Japanese trains, but German, British and American as well. Just to put such trains into perspective, a Union Pacific 4-8-8-4 Big Boy locomotive – the largest steam locomotive ever built with a length overall of 40.5 m – would be all of 90 mm long, while the width and height of a full size passenger coach is less than that of a pencil.



3 thoughts on “From 1520 to 3 – metric still rules (mostly)”

  1. I have had an interest in real and model railways since I was a lad. There was a railway line within sight of our house in the West Country so, of course I was a fan of the Great Western, or Western Region as it was in British Railways days.
    In our house was a loft just perfect for a model railway and I attempted to build a model railway there with some help from my father.
    I soon realised that my modelling skills were not up to my own expectations and I hated the look of OO-gauge trains running on skinny 16.5 mm track. I tried the EM gauge solution explained by John but the task of making my own track and modifying the axles of all the locomotives and rolling stock was just too much.
    John may well be correct that the larger size of model was to accommodate motors within the British loading gauge prototypes but I have always thought that it was because it was much easier to convert British original drawings. 4 mm to 1 foot is quite an easy task if the drawings are all in feet!
    Now the British modeller is stuck with this ridiculous situation and I have given up!


  2. Many model rail builders strive to reproduce real stations as accurately as possible. To put this into perspective, a Class 373 Eurostar electric multiple unit is 387 metres in length. A properly scaled model “0 gauge” would be 8.9 metres in length (working on a “7 mm scale”) and the HO while the “00-gauge” model (1:76) would still be nearly 5 metres long – far too big for a dining room table. Only the miniscule “T-gauge” (3 mm track) would result in a train that was less than one metre (86 cm) in length.

    Model railway builders who use the 4 mm to the foot scale do have one defence – they often work from originals drawings and most steam engines were designed using imperial units, British Rail’s last steam engine, the 2-10-0 Evening Star having been delivered in 1960. Britain’s metrication programme got fully under way in the decade that followed.


  3. Maybe if Hornby and Bachmann designed and built their respective ‘Thomas’ ranges in HO instead of utilising their OO models, they could put the U.K. on the same footing as the rest of the world and gradually encourage younger modellers to leave behind the narrow gauge compromise that OO always was. If HO was adopted in the UK, every model that was ever made in OO could be reproduced, and other similar dimensioned scales would probably adopt it as well.


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