This isn't exactly what you are after but it may help you derive the answer. My understanding is that it is 3.5 between the rails, inner edge to inner edge, but I would need to confirm.

http://www.cheltsme.org.uk/index-wheel_standards.html

This seems to confirm about the width between the rails:

https://www.homemodelenginemachinist.com/threads/3-5-gauge-specification.20158/

I guess it doesn't actually matter if you are running on wide rails or narrow ones, the only critical dimension there is how the wheels/flanges sit between the rails.

Double posting removed

Edited By Jim Nic on 20/12/2020 20:26:43

Treat it as 3.5" between the rail heads on the straight, depending on the radius curves you might be thinking of using then you might want to consider gauge widening of the track on the curves, the amount depends on the wheel base of the loco's you are intending to run. For example a short wheelbase 040 would probably cope with an 8' radius in 3.5" but a 6 coupled might need the gauge pushing out by up to 1/8" if you want it to go round the corner! For example a 6 coupled 5" gauge loco won't go round much less than a 30' radius curve without slight gauge widening. The back to back dimensions of your wheel sets will also govern points and check rails. Best to go with a standard, there are a few around certainly for 5" gauge, if you can't find one for 3.5" then you can scale it within reason.

Paul.

You can see why i don't post very often.

.

Don’t people use +/- dimensional tolerances where you come from ?

Malc’s question seemed perfectly reasonable to me … but I don’t do trains, so had no idea of the correct answer.

MichaelG.

Right, some numbers at last… And amplifying Jon Lawes answer.

My source is Martin Evan's Manual of Model Locomotive Construction, and differs little from the Cheltenham table cited by Jon. See his reference for a diagram.

3.5" track gauge : Min, curve radius 14ft, recommended min. 30ft.

Note that Evans does not go into track design so gives no figures for gauge-widening on curves, but a study of the dimensions and some drawing should give you a guide. The wheels are meant to run with a slight gap between rail and flange, on the straight, but obviously not overdone! So, this is what Mr. Evans gives

Wheel-set Dims, inches rounded to 2 dec. places

Back-to back (i.e. between the inside faces of the flanges): 3-9/32 = 3.28. (important for point-work and crossovers)

Wheel width (or thickness if you like): 13/32 = 0.41 (Cheltenham quote 0.375)

Flange thickness 5/64 = 0.08

Flange Depth (root to tip) 1/8 = 0.13

Coning (taper) 1 in 20 or 2.8º ( 5 – 6 º included. Don't exceed the angle)

That gives an outer flange dimension, or front to front as it were, of 3.12 inches. That does not account for the root of course, which takes up much of that 3/16 " a side difference

The thinner wheels the Cheltenham club specifies are probably nearer scale, but the Evans dimension may allow for more latitude in practical building of tracks. The Cheltenham society's track is of aluminium rails and their site says, for its longevity will not allow access to locos and rolling-stock with wheels not to specification. (I imagine other societies with similar rails would say the same.) These are all for models of Standard-gauge prototypes.

So – I am assuming you already have the loco and rolling-stock. I would first verify their wheel-sets' dimensions in case you need alter the track gauge slightly to allow for that, in proportion to the above dimensions. Be careful though because that may preclude future running of a locomotive built to standard.

Otherwise build your track to 3.5"g., but determine if you need widen that slightly on the curves, and by how much.

My own club has a ground-level 5 and 7.25 " g track, and uses two groups of setting-gauges each consisting of an aluminium bar with notches milled in it, one for the straights, the other for the curves. Two or three gauges are a placed a few feet apart on the track as it is laid. Ours is of flat bar, not prototypical section, but the principle is the same.

'

I tried to view the homemodelenginemachinist reference but it obviously contains something Very Naughty, because my BT "Parental Controls" blocked it! BT asked if I want access – yes – but then the page came up as " can't be displayed " . Good thing I am over 18.

Hi, I'm interested in railway locomotives and a general interest in railways and I do like a good railway journey, but as for track laying technical information, I know next to nothing, so I think the OP's question is valid. No question is ever stupid.

Regards Nick.

Bernie's question is indeed valid, and he and some of the responses show an interesting point.

There is plenty of information, hand-books etc. on building locomotives and rolling-stock, but I do not recall any similar literature on building the track.

I don't know how much there is the homemodel… reference Jon Lawes gives, but that's because my ISP and computer won't let me read it!

Yes, individual articles appear from time to time on such matters as making points, individual clubs' own track-building projects, HSE advice on line-side clearances… but a hand-book that collates the design side? Maybe one for the TEE 'Workshop Series' – that's the sort I have in mind.

I suppose the difficulty is someone with sufficiently broad experience of building railways from outdoor 16mm-scale up to 7.25 " gauge, permanent and portable, coming forwards to write it.

I might be prepared to help write one but as a technical writer, not a technical consultant. I have helped build fixed and portable tracks, but not design them. (Apart from a 7.25 " g oddity outside of model-engineering, and using wood and old bed-frame angle-iron!)

This is a link to the Bristol table of wheel and track standards, which itself is a reference to the SMEE standards of 1976.

**LINK**

But just because these are 'standards' does not mean that all published designs follow them – Martin Evans gives 13/32 for the with of the bogie wheels of 'Columbia', to give just one example.