Without generally agreed standards the coaches of trader A are likely to derail on the turnouts of trader B, and the locos of trader C to sit too high in front of the wagons of trader D. It should be expected that wheels from one manufacturer will run through the crossings of another if both are stated to conform to the same standard.

The introduction to the British Railway Modelling Standards Bureau (BRMSB) standards of 1950 is reproduced below, because it remains as valid today as when it was first written, probably by J N Maskelyne, then editor of Model Railway News and chairman of the BRMSB.

“In presenting these proposed standard dimensions to the modelling fraternity it cannot be too strongly stressed that they are only the recommended figures for all to use. There is nothing dogmatic about them at all, but they have been calculated to give the best possible trouble-free running, interchangeability within each gauge, together with considerations of manufacture – a very important point which the highly skilled amateur model-maker tends to overlook. … There is nothing whatever to prevent a modeller working to dead scale if he wishes, provided that he is prepared to operate on scale-radius curves with everything else in proportion. Likewise, a modeller can also adopt his own dimensions if he wishes to and is willing to operate his railway in isolation.”

In any set of track and wheel standards there are dimensions which must be held within close tolerances if trains are to run without derailment. The Guild recommends that modellers adopt a recognised standard, choosing that which best suits their personal requirements, even if this involves a degree of compromise. In this way reliable running is most likely, trade products can be used, and interchangeability with other lines enjoyed. This recommendation should not in any way hinder the development and eventual general acceptance of improvements in standards. Such developments have always been carried out by those modellers who, having the requisite skills, seek to improve appearance and running qualities. When the results of such work are incorporated in the products of the model suppliers and thus become generally available, the Guild must then consider whether this calls for additional standards or revisions of existing ones. Standards are not fixed forever, but are kept under regular review.

The drive to bring modelling standards closer to those of the prototype can only be successful if the interrelation between the dimensions of wheelsets and of track is properly understood and correctly applied. In presenting these standards, the Guild has, as its prime aim, the achievement of reliable running. A second objective is to afford the maximum of interchangeability with relevant existing standards. Throughout, it has sought to specify values for these interrelated dimensions which can be applied and maintained by typical modellers and which are at the same time conducive to economical manufacture.

As modelling in gauge O developed from the first commercial train sets to the present high standard, the dimensions of track and wheelsets moved progressively nearer to true scale. In consequence, there are at least twelve recognised dimensional standards for these components, although many are now obsolete and of interest only to collectors. In 1950 the BRMSB published two standards for gauge O, originally entitled O and OF. These became known as coarse and fine scale although more correct titles would be coarse and fine standards as the scale is the same. They are the immediate progenitors of the present Guild standards, with which they remain compatible. The modern nomenclature is used from here on.

At the outset, attempts were made to ensure that the coarse standard would satisfy the needs of those who ran vehicles with wheels both tinplate and solid turned on rails that were either tinplate or solid drawn. At that time it was not possible to produce wheels which would run on both coarse and fine pointwork. European, American and Australian modellers however, had adopted a different policy, disregarding the tinplate field and developing single standards lying somewhere between the two BRMSB standards and generally compatible with one another. Though wheelsets complying fully with these three standards are not compatible with pointwork made to either BRMSB standard, some manufacturers of ready to run rolling stock have, by slight modifications, been able to produce wheelsets able to run on track made to any of the five standards. Later, the BRMSB introduced a unified standard which, despite its name, was not fully compatible with any of the foregoing other standards, including their own. The ScaleSeven standard, despite ancestry going back to the turn of the last century, was never adopted by the BRMSB.

The BRMSB ceased to exist in 1960 and their associate body the META (Model Engineering Trades Association) some fifteen years later. Consequently their published tables in due course went out of print. The committee of the Guild was then asked to reissue them for gauge O which it did in 1975. However, in the later eighties when a reprint was required, it became clear that the developments of the previous twenty years called for some revision. In 1989 therefore, following extensive research and development by the Technical Committee, revised fine and coarse standards for gauge O were published. These, while remaining fully compatible with the original BRMSB ones, removed certain anomalies in tolerances, thereby making it possible to include dimensions for a true universal wheelset.

The fine standard was quickly established as the one used by a majority of modellers and has benefitted from considerable trade support. Some modellers considered that fine was not fine enough, and preferred ScaleSeven, which was based on prototype dimensions for wheels and track scaled down to 7mm/ft, and this was later adopted by the Guild as a standard.

At the same time, the fine standard was criticised for the large difference between the track gauge and the flange-to-flange dimension of the wheels. This stemmed from the desire to maintain some compatibility between the fine and coarse standards, but it did allow locomotives and rolling stock build to fine standard dimensions to “track” or move sideways. In addition, at the extreme dimensions allowed by the standards, there is no support for a wheel passing through a rail crossing. This leads to wheels dropping in the crossings and to uneven running through points. In an effort to achieve more prototypical running, slightly reduced gauges of 31.5 mm or 31.25 mm were introduced. The wheelset dimensions and turnout check and wing rail clearances were kept the same. These variants have now been adopted by the Guild as O-MF (O-Medium Fine) and O-SF (O-Super Fine) track standards.

Scale is the relationship between the size of a model and its prototype. The British scale is 7mm = 1ft, which is a size ratio of 1:43.54, usually rounded off to 1:43.5. It is not, however, recommended that this rounded value be used for critical dimensions because unacceptable errors could result.

In North America the National Model Railway Association (NMRA) recommends a scale of ¼in = 1ft, which gives a size ratio of 1:48. In other European countries, ratios of 1:43.5 and 1:45 are used, the 1:45 ratio being recommended by MOROP, the European coordinating body of national model railway associations.

The gauge of model track was originally defined as the distance between the centre lines of the rails. This was a manufacturing convention, used when rail was formed from tinplate. Gauge O was an addition to the original sequence of gauges, inserted below what was previously the smallest, Gauge 1. By this convention Gauge O was 35mm between centres, which happened to give a between-rail gauge of 1¼in. It was left to Henry Greenly to select a modelling scale for Gauge O. He chose 7mm = 1 ft as the best compromise between gauge and scale for that time. In earlier days the tightness of the British loading gauge caused problems for those modelling British prototypes, but these have largely ceased with the advent of miniature electric motors.

The gauge is now defined as the distance between the inner faces of the rails. In Gauge O, like the scales, this varies slightly. In Britain and Europe the standard O gauge is 32mm, whereas in America it is 1¼in (31.75mm). Before 1950 and the BRMSB standards, the British standard also was 1¼in. In some standards the gauge differs from 32mm, as noted in the current Guild standards, and in Other track and wheelset standards for historical British standards and those of other countries.

Some critical track dimensions, particularly the minimum radius, the width of wheel tread and the clearance between wheel flanges and rails, cannot be scaled exactly without imposing severe limitations on the scope of the planned layout. The minimum curve on British passenger lines is usually 160m (8 chains) which, in O Gauge, is 3.7m (12ft). The minimum for locomotives other than industrial and then only when running dead slow is 90m (4 chains) which is 2m (6ft). On most model layouts the curves are necessarily much tighter than the prototype, so, on the severest curves, it may be necessary to widen the track gauge or allow more side play for certain of the coupled axles on longer locos, or both. These modifications are easier to contain within overall scale dimensions when the gauge is reduced below the scale dimension, so that the wheels are closer together, allowing greater sideplay. The Guild fine and coarse scale standards have this feature. The ScaleSeven standard, however, uses a virtually exact gauge. In consequence, a larger minimum radius is likely to be necessary than for other standards.

The British scale of 1:43.54 coupled with a gauge of 32mm applies only to models of standard gauge prototypes. The builder of rolling stock running on other gauges is free to choose either to retain the 1:43.54 scale and change the gauge, or to retain 32mm gauge while altering the scale. These variations do not form part of the Guild standard.

The Guild standards for track and wheels

Other track and wheelset standards

Scale and gauge equivalents of prototype gauges

This article was originally compiled by the Technical Committee for the Gauge O Guild Manual. It was updated and adapted for the GOGWiki by Nick Baines.