M Menu
b 0 Items s

SWheel - Rail Interface (Back to Back)


2890 posts

In 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?  Undecided


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.  Frown


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. 


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.  Smile


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.  Smile


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!   Wink

I enjoy life - it gives me something to do! - especially modelling BR's Southern Region 1955-61.


2348 posts

Community Moderator

Thanks Caiptean, I've always understood that B2B can be a problem but your clearly written explanation has helped me to understand the mechanics of derailment a lot better. R-

OO Gauge : RM Pro : Elite (v1.42) : Dell Laptop W10 : 9 Locomotives


8860 posts

 Wise words indeed.


A useful tool is a Vernier gauge, you can get one with digital read-out fairly cheaply so you don't have to squint at little etched lines any more. This will enable you to measure the back to back very easily and you will undoubtedly find other uses for them.

Green trains are best!


2890 posts

@Roger B



Thank you both - reading across several topics it was rapidly becoming clear that this needed to be a subject in its own right. The mechanics of the wheel-rail interface are often not well understood and whilst LC&DR and myself understand these (from our railway-industry perspective) it is a subject that needs to be set out with clarity for all railway modellers.  Smile

I enjoy life - it gives me something to do! - especially modelling BR's Southern Region 1955-61.


8860 posts

 The wheel rail interface was always a troublesome issue for the railway industry, and one that has occupied many learned minds over the years. Derby Research was set up very much as a means of getting to understand it and in the 1960s there was a great deal of experimentation to improve this aspect of comfort and safety.  The French and Japanese were also highly active in wheel/rail interface research.  The APT depended in getting this exactly right and the Shinkansen and TGV likewise.


I attended a number of accident investigations, and was fascinated to hear the Derby Scientists explaining how the interface was so important. One element which fortunately doesn't affect model railways all that much, was 'Cyclic Top'. This was a wave form that jointed track adopted which had peaks and troughs over a 60 foot rail, caused as the wheels hammered the joints into the formation underneath.  1960s period tank wagons were falling off plain line track fairly frequently and it was learned tgat at certain speeds the 15 foot wheelbase passing over these waves set up a periodic osscilation which eventually caused instability and the wagon 'bouncing' off the rails. Simply relaying the track even with continuously welded rail was not sufficient to remove the underlying problem, and  a short term measure was to impose a speed limit that avoided the harmonics, and long term to dig out the formation and blanket it with new material before relaying.


All this goes on 'behind the scenes' but is what made working for the railway such an interesting experience.

Green trains are best!


9710 posts

Community Moderator


This heterodyning where one frequency beats against another frequency to produce coinciding intermediate frequencies resulting from the sum or difference of these beating frequencies occurs in many situations, including radio waves where it was 'invented' (remember superheterodyne tranny radios).


Roads made from cast concrete blocks of the same size will cause lorries to tramp over the joints which can turn to bounce same as your wagons, ships will rise and plunge cyclicly according to their length and wave swell pitch. Even clouds do it whereby a band of one cloud type will roil in position whilst other clouds skim past.


A sideways version of this is if you try to tow many trailers such as baggage trolleys, etc as the speed increases a snake occurs which gets worse until the whiplash upturns the whole train.


Back on topic...

Model rail wheel form is defined in RP25 and NMRA has good coverage of rail form also. They also cover Hi-rail for use with large flange wheels.

Halton Brat - Running Win 10, 64-bit - RM (Pro-Pack) with Elite as Controller-A, Select as Walkabout and E-Link as Controller-B - Locos are mostly TTS. - http://myweb.cytanet.com.cy/honnor/


2890 posts


Links to NMRA standards most useful to this forum subject and its readers - thank you for posting these - it is much appreciated!  Smile


In terms of LC&DR comments on cyclic top (we both encounter these professionally) and yours expanding on harmonics would certainly be a interesting subject of discussion in the off topic lounge  Wink

I enjoy life - it gives me something to do! - especially modelling BR's Southern Region 1955-61.


5769 posts

Just a side-comment - without harmonics, your radio wouldn't work!

'Beating' one frequency against another is how the radio waves are converted to sound waves (putting it VERY crudely!)

if it works first time, you did something wrong!

Hi there,


as I understand things and confirmed by my son( a motor vehicle technician) suspension is on vehicles to keep the wheels on the road, and the spring rate and damping tuned for passenger comfort.  I don't think that at the speed that our models ron it will be inportant, but is it , has anyboby done any experiments with suspension on model railway rolling stock?




2890 posts


.. has anyboby done any experiments with suspension on model railway rolling stock?



I presume you are thinking in 4mm and the answer is a guarded yes. However, suspension is very difficult to achieve /adjust so many opt for compensation which ensures the unsprung weight of the locomotive keeps all the wheels sitting firmly on the railhead.


Suspension is usually the preserve of P4 modellers although I have some sprung Mk1 coach bogies (Bill Bedford kits). However, I general use MJT carraige compensation units for some of my emu fleet. However, in general most of my fleet has neither. http://www.dartcastings.co.uk/mjt.php#SuspensionSystems-CarriageCompensationUnits


Some of my kit-built locomotives use compensation units - for example the D-class 4-4-0 is essentially a Bo-Bo configuration. The front pony (unpowered to the term Bo is technically incorrect) pivots and radially-rotates as a bogie should. However, the driving wheels are at either end of an equalising beam which pivots in the centrally between the two pairs of drivers (obviously this do not radially-rotate). The locomotive weight is therefore transmitted down through two points - the centre of the pony truck and the centre pivot of the equalising beam with all wheels on the railhead at any point in time.


Rather than my rambling on you may wish to read http://www.clag.org.uk/41-0rev.html  In terms of springing in commercially produced models it is probably (for the most part) not commercially viable or considered necessary.


I hope this helps - Caiptean  Smile

I enjoy life - it gives me something to do! - especially modelling BR's Southern Region 1955-61.

Please create an account or login to reply.

Forum Rules

  • The Hornby Forum is intended for discussion of all things model railways. Primarily a place for newcomers to ask questions and seek assistance from like-minded individuals, the Hornby Forum offers a user community for all those interested in railways.
  • Discussion of non-Hornby brands is allowed, however, active promotion or advertising of these brands is not permitted.
  • Please keep in mind that the Hornby Forum is a publicly viewable space and you should never post personal information (including email addresses). 
  • While every effort is made to contact you before any censorship, we reserve the right to amend or remove any content without explanation.
  • All customer service enquiries should be directed to Hornby Customer Services.

Useful Links

Forum Guidelines

Membership Restricted Product