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Size question

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DollyDigger20/12/2020 11:34:04
51 forum posts
7 photos

Just to help me sort out sizes for the garden i am playing with drawings but i really want it to be accurate, long story but might tell at the next online mtg

What i really really need is the precise dimension of the 3.5" track so i can make a track gauge, or am i not allowed to do that?

Many thanks, Malc

old mart20/12/2020 17:59:05
2669 forum posts
176 photos

Could it be 3.5"?

Jon Lawes20/12/2020 18:48:38
469 forum posts

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.

This seems to confirm about the width between the rails:

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.

noel shelley20/12/2020 19:49:59
356 forum posts
9 photos

An interesting point in this vein is that our Standard Gauge was originally 4' 8" but was found to be too tight and hence the extra 1/2" ! This was also true for The GW Broad gauge which was in fact 7'1/4" for the same reason !


DollyDigger20/12/2020 20:10:59
51 forum posts
7 photos

Well at first i thought people are going to go well its 3.5" doh, but since when in life has any size that has been quoted is correct to that size? i have 2 size 9 shoes that are different :-D
so i am taking 3.5" to be inside the rails and nothing to do with wheel track and also my question i suppose is 3.5" is it 88.9mm or is there an accepted figure allowance of that?

@Jon, thats mint, you have no idea how long i spent trying to find that type of info, brilliant thank you.

@noel, i wonder what dictates too tight with 4'8", is that the radius curves ? surely you could narrow the wheel track,

need to read on that, new knowledge.

@old mart, :-D i wasnt trying to be funny just maybe should have put some more text behind it, cheers all.

old mart20/12/2020 20:17:15
2669 forum posts
176 photos

3.5" is 3.5", what ever more would you want, any other answer to your stupid question would be superflous.

DollyDigger20/12/2020 20:23:59
51 forum posts
7 photos

Wow thats strong words no wonder beginners dont want to ask on forums anymore, some people feel so above us !!!!!!!!!!!

There was nothing wrong with how or what i asked un my mind, having just read the links from jon i am amazed at how important the dimensions are to what was before today a simple wheel.

I really truly hope you never get the need to ask a question to any one about something you dont understand, and dont forget dyscalculia might not be in your blood but it is in mine, should i really have to announce that?

Edited By NUFCBernie on 20/12/2020 20:24:17

Jim Nic20/12/2020 20:25:41
295 forum posts
171 photos


Whatever happened to "There's no such thing as a stupid question"?


Jim Nic20/12/2020 20:26:11
295 forum posts
171 photos

Double posting removed

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

Georgineer20/12/2020 20:34:06
488 forum posts
30 photos


It's so nice to be on a forum where people are nice to each other, isn't it? I've heard tell of other forums where people are sometimes quite rude.

George B.

Paul Kemp20/12/2020 20:55:22
600 forum posts
18 photos

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.


john halfpenny20/12/2020 21:01:57
128 forum posts
21 photos

Bernie, I thought your question quite sensible. The very intemperate response deserves a forum ban.

Phil super720/12/2020 21:19:12
14 forum posts

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

Peter G. Shaw20/12/2020 21:36:20
1244 forum posts
44 photos

Hello Malc,

I don't run locos of any description, but in an attempt to make up for the rudeness shown earlier, may I point you at Tubal Cain's Model Engineer's Handbook, specifically pages 13.15 to 13.18 where he discusses track & wheel standards. For 5 inch gauge, he shows back to back wheel standard of 4.687 inches or 119 mm. There is a lot more as well, but nothing that states that the track standard is anything other than 5 inch. or 127mm.

Hope this helps,

Peter G. Shaw

Michael Gilligan20/12/2020 21:42:24
17286 forum posts
778 photos
Posted by old mart on 20/12/2020 20:17:15:

3.5" is 3.5", what ever more would you want, any other answer to your stupid question would be superflous.


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.


Michael Gilligan20/12/2020 22:24:15
17286 forum posts
778 photos


Here’s a tolerance for ‘full size’ track:


The US standard railroad gauge is 4 feet, 8.5 inches (Gauge means width between the two rails). The U.S. federal safety standards allow the standard gauge to vary from 4 ft 8 in (1,420 mm) to 4 ft 9 1⁄2 in (1,460 mm) for operation up to 60 mph (97 km/h).

Ref: **LINK**

... Presumably there must be something similar relating to 3.5” track, although I doubt if it’s directly scaled.

Please don’t give up on us just for one member’s rudeness.


Nigel Graham 220/12/2020 22:25:42
1052 forum posts
16 photos

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.

Martin W20/12/2020 23:24:40
866 forum posts
29 photos

O Mart

I would think that an open apology would be in order after such an acidic post - perhaps the old advice of " If you haven't got anything good/constructive to say then keep quiet ". As others have said it was a perfectly reasonable question as tolerances can vary and the given measurement/dimension may only be valid under certain conditions i.e. straight runs of track or whatever.

Martin W

Nicholas Farr21/12/2020 00:24:07
2621 forum posts
1225 photos

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.

Peter G. Shaw21/12/2020 08:54:13
1244 forum posts
44 photos


It seems I made a mistake by thinking it was for 5 inch gauge when it's actually for 3.5inch gauge. Nevertheless, the reference I gave still stands although the figures I gave are irrelevant for this question.

Sorry about that.

Peter G. Shaw

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