Model Railway
Wheel-Rail Interface
(aka Back-to-backs)

by Caiptean


These help notes are for the benefit of railway modellers.

They were originally prepared for members of the Hornby Forum, but equally apply to other makes.


Wheel-Rail Interface

On all railways the wheel-rail interface (in industry parlance) is the crucial element in ensuring trains run. However, its importance can often be overlooked or even misunderstood by some railway modellers – including manufacturers. Standards used on railway models vary considerably (even within a manufacturer’s own range) and this can lead to issues and poor running especially on turnouts and crossings.



Many modellers will have a turnout on their layout upon which a number of items of rolling stock regularly derail. In the end that turnout gets lifted /replaced, the problem appears to go away and the conclusion that it was badly laid is arrived at.

Except was it?

Obviously uneven track will lead to derailments but so will defective wheelsets. I don’t mean wheels that have chunk missing out of them; just defective in their profile is poor and their back-to-back dimension too tight.

When passing through a turnout’s common crossing there can be a tendency for wheels to try and take a diverging route either jumping as the wheelset catches the crossing nose or simply derailing. Wheelsets with tight back-to-back (i.e. less than 14.5mm) have considerable sideways play on the track as the flanges are not sitting snugly within each running rail; this excessive lateral play between the running rails can often lead to poor running.

With this excessive lateral play the rail vehicle itself can also move more than it should. So, it struggles to follow the track and at certain locations the tolerances required to keep the wheelset on the track are exceeded leading to a derailment. Often this may happen with (say) two or three wagons which are then labelled as being ‘rogue’ or troublesome and so are ‘carded’. Relegated to the back of the layout they are now rarely used without further investigation. 


Model train wheelsets 

As a railway modeller my layout has seen more visitor-mileage than with my own stock and this provides an interesting insight into what occurs with models. The coarse wheel profiles of Lima and Jouef cannot run; neither can the older Triang and Hornby wheels but much of the rolling stock today can. Moulded plastic wheelsets are taboo for similar reasons.

I know the track is spot on – many hours of running my own stock fitted with finescale wheelsets amply demonstrates this. But then (say) that pesky leading axle on a visiting Bachmann class 40 occasionally derails on the common crossing of a single slip. Up-turned on the modelling desk it is found that the back-to-back dimensions of that ‘pesky’ wheelset was less than a millimetre too tight and once gently opened out to 14.5mm the locomotive runs faultlessly thereafter. 


Excess lateral axle play 

Many model locomotives also have excessive lateral play in their driving axles and for a layout where 3’ is the minimum radii (a standard used by a number of us) this also leads to unreliable running. Wheels are removed and (for a 2mm axle) 8 BA brass washers are inserted to reduce the sideplay; the locomotive’s running is significantly improved. 

Of course, there are many who rarely encounter these issues and may even be bemused by them. However, next time you have a derailment just get a set of callipers out and check the dimensions of the derailed vehicle’s wheelsets – you might just be in for a surprise! 


Back-back gauges

Besides using a Vernier gauge several manufacturers make back-to-back gauges.

GW Models of Lancing, West Sussex make an excellent adjustable back-to-back gauge primarily intended for steam locomotives
(mail-order but not on-line)

DCCconcepts produce a number of gauges including both the standard 14.5mm, ‘fine 14.75 and ‘special fine’ 14.85mm gauges.

Both C&L and Markits produce standard 14.5mm back to back gauges.

The standard ‘00’ back-to-back gauge is 14.5mm. However, although finescale modellers may use 14.75mm /14.8mm /14.85mm gauges these are not recommended for proprietary rolling stock.


Checking the Back-to-Backs
by LC&DR

Modern ‘00’ wheels should normally be 14.5mm between inside faces and if narrower that that will cause derailments, especially at turnouts (points). It is possible to buy a special gauge to measure this, but it is perfectly possible to use a cheap Vernier gauge instead. Wheels found to be out of adjustment can usually be adjusted by gently sliding them on the axle. Locomotive driving wheels should only be tackled by experienced modellers.

Checking back to back with a Vernier Gauge

©Image LC&DR


Further reading

There are defined wheel contours for RP25 and NMRA recommended rail practices.

The Double O Gauge Association also publish their standards.

NMRA also has a recommended practice for wagon weight.

The Scalefour Society publish on springing for railway models (very specialist).


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