Wheel & Rail Standards

A railway is a system, more than a collection of related disciplines, and there are many places where engineering disciplines interact. An obvious interface is that of loading gauge and structure gauge. This simply means that the civil engineer and the rolling stock engineer have to agree on a set of standards that ensure that no vehicle can come into contact with a bridge or platform edge or any other item on or about the lineside. Another very important interface is that between wheel and rail, which is vital for the smooth running of trains and for their safe guidance along the way.

At first glance, there seems to be a confusing variety of standards for 4mm scale modelling, with OO, EM and P4 or S4. EM and P4/S4 standards are now well defined with a growing commercial following but, to be perfectly honest, wheel standards for OO are in a bit of a mess. I put this page together in an attempt to understand the choices and my options. The main RTR manufacturers have raised their standards significantly since the early days but that leaves us with a heritage of different standards. I have a substantial fleet of Hornby Dublo stock with their original wheelsets which defines my starting point – it would be tedious to replace them all. However, I’m becoming convinced that plastic wheels are dust magnets, metal wheels stay clean longer so there’s no dirt to transfer to the rails.

There are a number of considerations and the dimension most often cited as critical is the wheelset back-to-back measurement. However, this dimension is bound up with two other key dimensions: the flange width and depth. These have to match the flangeway widths of the turnouts on the layout, else there will be trouble. As for the full-sized railway, the critical dimension is the checkrail gauge because this governs the safe passage of the wheelsets through the common crossing of a turnout. Correct dimensions and clearances here ensure that the checkrail does its job of holding the wheel flange just clear of the crossing nose.

The Double O Gauge Association has published two standards: Fine and Intermediate, and Finescale OO has also published a series of pages on the subject. These are set out in the tables below. I measured a sample of my Hornby Dublo metal and plastic wheelsets and compared them with a sample of current Hornby and Bachmann wheelsets from my fleet. They are pretty much the same, so Meccano Ltd was ahead of the game 50 years ago!

The track standards I can adopt will be dependent upon the wheel standards I decide to accept. The words of John H Ahern, a significant inspiration to railway modellers ever since he came to the fore in the 1940s and 50s, are worth remembering: “I don’t want to spend all my time messing about with microscopic wheel and track adjustments. I want to make models, models of all kinds of things.” To that I would add: “ ... and watch the trains go by.” Thus, the rest of this page must be taken as an exercise in trying to understand what’s out there and not as a statement of intent.

Wheel Profile   Wheel width Flange width Flange depth Hornby Dublo 3.4 1.0 1.2 OO Intermediate 2.75 0.76 0.64 OO Fine & EM 2.29 0.51 0.69 P4 2.000 0.353 0.366 All dimensions in millimetres. EM & P4 included here for comparison. There is an apparent discrepancy in that the flange depth quoted for OO Intermediate is less than that for OO Fine and EM. The published drawings give these dimensions to a thousandth of an inch for OO Fine and EM and to four decimal places for P4, which I have converted to millimetres. I remain unconvinced that such accuracy can be achieved in the real world!

Wheel-Rail Interface

Cross-section through a common crossing, the critical location for guidance and clearances.

All dimensions in millimetres. Wheel Check Gauge and Clearance dimensions calculated from other values.
My suggested track standards for Hornby Dublo wheels are derived by extrapolation.

All this presumes that the track gauge is maintained at 16.5mm. It’s worth remembering that British Rail adjusted the gauge from time to time and the 4' 8½" is only a nominal value. This link is to a comprehensive explanation of the different track gauge and wheel standards that have been established. It’s a huge subject that seems to have generated more heat than light but, I believe is very well worth a dispassionate read.

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