Track & Wheel Standards

Overview / Introduction

Note: if you really want to learn and understand, then read all of this. If you want a simple answer, first, there is not one, but you can read my "conclusions" section below, it's my opinion. It's supported by facts, but it's my opinion.

Being an engineer, I am constantly investigating the factors that will make my railroad run the most reliably. So, naturally I looked for standards, and I have tried to make sense of the ones I have found.

Before diving into numbers, a small explanation of my focus, since there are a lot of different SCALES that use "G" gauge track, 45 mm.

I model large scale standard gauge. Since most of the available standard gauge rolling stock is available in 1:29 scale, I will focus there. (Yes, I know that 1:29 is not "purely correct" for the scale/gauge combination.)

Please realize that there are many different scales that use "G" or #1 GAUGE track, 45mm. Some are prototype standard gauge (1:32) and some are narrow gauge and the scales are quite varied. For example, "G" gauge track is appropriate for 1:32 standard gauge modeling. Also, "G" gauge track is appropriate for narrow gauge modeling, but the resultant scale for the track to represent 3 foot narrow gauge yields a rolling stock scale of 1:20.3.

What has happened (as in many scales) is that with the track generally available in one physical gauge, you can use the same track gauge to represent different scales. In the prototype, i.e. 1:1 scale, this resulted in different gauges.

But different scales require different size and spacing of ties, and different rail "height", representing the different "weight" of the rail in the prototype which was sized to the weight of rolling stock and usage. In the hobby, we represent this by saying:"code xxx" where xxx is height in thousandths of an inch, for example code 332 is 0.332 inches tall.

OK, so maybe all of this is boring so far, but there is some relevance here: the scale you use can affect the standards you follow. Also, indoor modelers can utilize standards closer to the actual prototype dimensions.

So, I will focus on 1:29 scale, standard gauge, used outdoors.

Who has standards?

Well, as far as I can tell, there are only 3 groups recognized in the world that have standards applicable. The NMRA, the G1MRA, and MOROP (who administer the NEM standards).

Which is better? Hoo boy, that's an endless debate! So let's try to get an overview of how these are organized.

NMRA standards:

This is the place that most people would "start" if they came from other scales and were in the US. The standards are organized in sort of a top-down approach, so I start with the most basic and overall standards, and then go "down" in increasing level of detail.

Please note that while there are usually 3 sets of standards for each category, not all are applicable to 1:29 scale outdoors, so I list all the applicable standards, both approved and proposed. These are identified below.

CLICK HERE  for the main NMRA standards page.

Standard S-1 General Overview - 7/2004

This is the entry point for standards. It defines the structure of the standards, and defines the terms:

  • Track Gauge - distance between railheads
  • Track Check Gauge - distance from flange side of a guard rail to flange side of a frog
  • Wheel Check Gauge - distance from the back of a flange to the tread side of the other wheel
  • Span - distance between flange sides of guard and wing rails
  • Flangeway Width - distance between flange sides of wing rail and frog
  • Flange clearance - vertical distance from railhead to highest obstruction below it
  • Flange depth -actual depth of the flange on the wheel
  • Switch point spread - detailed in the document.

If you are not an expert in track (and even if you think you are!), I strongly suggest you read this specification (linked below). It is only ONE page! It gives you the the understanding of the critical relationships between these elements, has nice drawings, and explains how and why guardrails keep a wheel from hitting the point of the frog, how a wheel traverses a frog without dropping in, etc. Turnouts are the #1 cause of derailments in model train layouts.

CLICK HERE to view NMRA approved standard S-1.

Standards S1.1, 1.2, 1.3 scale and basic "definitions" of classes of scales: 7/2009

OK, now we are on to the more detailed standards. Before we go there, some explanation is necessary. The standards that follow now refer to the NMRA's idea of 3 different classes of modeling:

Proto & Fine Scales (where wheels & track are very close to exact scale) (NMRA standard S1.1)

Standard (where wheels & track dimensions are somewhat "coarser" than exact scale - "standard modeling") (NMRA standard S1.2)

Hi Rail (origin in the "high rail" of toy trains, our "Garden/Outdoor railroads" fit here in the NMRA's "mind") (NMRA standard S1.3)

To put these 3 groups in perspective:

  • Proto & Fine Scales is basically prototypical, i.e. the exact scale representation of the real thing. Prototype dimensions are just flat impossible outdoors. The Fine scale is not even defined for any G scale. Forget it.
  • Standard is where you find the "standard" scales. If you buy an average HO loco off the shelf, it will be following the Standard scale practices. This standard mentions "LS" with varying scales on "standard" LS track, and the same track gauge as Fn3 (1:20.32 scale)
  • Hi-Rail (now called Standards for Scales with deep flanges) is pretty much the "toy" scale category, The July 2009 approved Standard has LS here also. This is toy train stuff. Even the track gauge is relaxed 0.004".

For those of us in "G" scale outdoors, the most recent changes is that Large Scale is now also in the STANDARD scale category,

Another BIG change is that the Hi-Rail standards have been refined from being totally toy-like to something that will work better outside AND will allow long trains and better operation.

Standard S-2 - couplers

New, and gives the coupler height and tolerance for various scales. It's 1.063" for 1:29 & 1:32. It's 1.125 for the other "G" scales. It even specifies height of hook and loop (0.75")

click here for the S-2 standard

Standards S-3.1, S-3.2, S-3.3 - Trackwork

Again we have three standards, depending on if you are Proto, Standard, or Hi-Rail. All of these were revised in February 2010.

In my opinion, the new standards are good news, the Hi-Rail standards have been revised to much more reasonable specifications. For example, the 2004 standard has a guard rail flangeway width from 0.106" to as large as 0.131, and a flangeway depth of 3 mm. (.118"). The standard notes that the flange depth of 3mm is to accomodate deep flanges and results in flange-bearing frogs. (Yuck)

The early 2009 standard has a specification of 0.104", with a tolerance of  - 0.002 and + 0.007  ( 0.102 to 0.11 ) much more reasonable in my opinion, and keeps the 3mm minimum flange depth.

The July 2009 standard tightened this up again.

Unfortunately, the February 2010 standards go backwards, with a specification of 0.115, +0.002 and  minus 0.023". Obviously what has happened is to set a "target" properly, but "Grandfather in" the existing manufacturer's toy like switches. This also implies sloppier and narrower back to back spacing on wheel sets (another historical situation with Aristo and LGB)

In my experience, using closer tolerances (similar to these new ones) combined with a better back to back setting on wheelsets, has VASTLY improved my ability to run more reliably through switches.

I think most people (and manufacturers) will use the old "Hi-Rail" standard 2/2010 "Deep Flanges" standard - S-3.3  - for deep flanges. The flangeway depth is minimum 0.118 (3 mm). This standard allows for really deep flanges. There are also some other exceptions on this page where back to back can be decreased to 1.542" and the wing rail flangeway increased to 0.131". I think if you read between the lines here, EVERYTHING probably meets this standard, and the exceptions really take you back to the "wild west" of G scale. Basically with the exceptions and sloppy and asymmetrical tolerances everyone is "grandfathered in" with no changes in production needed. (Yuck). There are significant compromises involved in using this standard, both in appearance and operation. This is for toys.

I use the 5/2010 "Standard" standard - S3.2 - if you read this, you see that other than minimum flange clearance, this standard is the same as the 1/2006 Proto 20.3 standard - S-3.1 Now we are getting somewhere! The tolerances make sense.  But the same minimum flangeway depth is just downright stupid. This is because it's difficult to make "proper" frogs in G scale, and flange-bearing frogs have their own problems. More on this later, but it seems another political attempt to "grandfather in" deep flanges to keep the manufacturers "happy". This sucks.

Also there is an error in the standard. S3.2 says:

Due to the inherent nature of large scale trains, the wheel and track standards for “Standard” (Sx.2) and“Deep Flange” (Sx.3) are identical except in terms of  flange width and depth, thus the track H depth also is changed.
Well, both standards say flangeway depth is 3mm minimum, the SAME, NOT changed. Probably a mistake, but no one fixed it.

Standards S-4.1, S-4.2, S-4.3 - Wheels

Again we have 3 classifications, and again, the Standard and Hi-Rail standards have been updated in February 2010.

I will focus on the back to back specification, and only in the Hi-Rail standards as the new standard has another improvement in it that makes a lot of sense:

In the new approved 1/2015 S-4.2 "Standard" standard: click here for the 4.2 standard (Note that the large scale standards are on the last 2 pages)

  • back to back gauge is 1.575" +.019, -.005, or a minimum of 1.570" and a maximum of 1.594"
  • flange thickness is 0.059" + 0.002" -0.018.
  • max depth of the flange is 0.066 (1.68 mm)  (what sense does this make with a 3 mm MINIMUM flangeway?)

In the new approved 2/2010 S-4.3 (old "Hi-Rail") standard "wheels with deep flanges": click here for the 4.3 standard (Note that the large scale standards are on the last 2 pages)

  • back to back gauge is 1.575" +.019, -.015, or a minimum of 1.560" and a maximum of 1.594"
  • flange thickness is 0.059" + 0.002" -0.018. 
  • max depth of the flange is 0.118" (3 mm)

There is also a note that says if you use flanges greater [thicker] than 0.076" you should reduce the back to back, but stay within the check gauge, and this is highly recommended. What crap this is, basically messing with the back to back makes your operation through turnouts worse, and this is the #1 cause of derailments. You CANNOT have sloppy specifications and loosen up the guard rails and wing rails and get smooth operating switches. Another cheap sell out to grandfather in poor wheel standards from manufacturers.

NMRA standards conclusions:

The standards approved are still sloppy and have too much slop in them to grandfather older equipment and poorly manufactured equipment.

I'm disappointed by this, so my opinion is to follow the target values for the "standard" standards, not the proto/fine and not the deep flange standards. Even then you should not use the huge, sloppy tolerances given. Also the flange depth is nuts for outdoor use. 2mm deep flanges are tough enough, don't try 1.68mm.

G1MRA Standards

This is a favorite standard for people in England, and many narrow gauge modelers here in the US. It is my opinion that the 1:20.3 people here in the US are much more concerned with scale and gauge than the 1:29 modelers. I'll only include the "WHEELS AXLES AND TRACK, STANDARD GAUGE 1", as the fine scale standards are not applicable here.

Click here to see the G1MRA specification

The back to back is 1.574" +0.020 -0

the flange depth is max is 0.079 (more sense than the NMRA 0.068) although they state you can use 1.5mm 0.059 if you want for sprung/equalized trucks. I don't know anyone who can do this outdoors.

flange thickness is 0.060" +0 -0.020

MOROP / NEM standards:

This is very popular in the areas outside of the US and England. NEM is the name of the standard, and MOROP is the group that works on the standards. (So technically, there are no such things as "MOROP standards", only NEM standards.) 

Unfortunately the official site has no English translation, here is a link to the applicable standard (a new one just for "garden trains": click here for NEM 311G.

Thankfully H.J. Mueller translated the 310 standard for me: click here for a translation of NEM 310

(note, I also need to also check out NEM 313)

Note: The MOROP standards are not available in English. Here is the root web page for all of their standards: so you have to stay in German to navigate to the standards pages, if you are in English, all you get is a page of apology for not translating them... sheesh, how long does this take?

Use Google Translate to translate the page with the links, but then open the native French web page in another window/tab to open the standards and look at the .pdf files. If you need to translate the .pdf files, click on the links on the Google-translated page. You get the text, but not the illustrations. Note also that what MOROP calls "large scale and garden railways" is not us. Those standards are for the ride-on scales.

My bottom line:

  1. stick with the NMRA "standard" standards, 3.2 and 4.2
  2. BUT use the target values (see below), and not allow the super wide tolerances given there to grandfather in the manufactures that made huge flanges, flangeways, basically not much better than toy trains.
  3. The flange depth of under 2mm is nuts, run 2 to just short of 3 mm flange depth, too many of our locos do not have adequate suspensions.

The G1MRA standard can be followed and everything will be fine, except the flange depth.


I want a "workable" set of 1:29 scale standards, that improve the quality of operation, and have some chance of being used on current production 1:29 rolling stock. This means there has to be some "reasonable" route to make Aristo, USAT, Bachmann, and other 1:29 stuff conform to the standard, also track, especially turnouts.

This is what I want to avoid: (the combination of back to back and the flangeways actually has ripped wheelsets apart in some cases!) If it is not obvious, the back to back spacing is so under gauge it's lifting the wheel treads off the rail head. I know that there is check gauge which tries to combine wheel gauge and back to back, but the width of our wheel treads is so wide, that all you need is a proper back to back and a flange of the proper thickness and everything works out.

The proper flange thickness also allows you to have nice reasonable flangeway widths, so besides looking toylike, it allows the wing rails and closure rails to work properly around the frog, supporting the wheel, so the wheel does not fall into the frog.

To make this clear, once you have sloppy back to back tolerances, you have to increase the flangeway widths so much the wing rails and closure rails cannot support the wheel tread through the frog, and the wheel drops into the frog with a bump. LGB solved this with flange beaing frogs, that is, the edge of the flange rides on the bottom of the flangeway around the frog.

This works, as long as you have huge flanges, and ALL the SAME depth. 

I hope you are getting the message that all this stuff is inter-related, and once you loosen the tolerances in one area it has a ripple effect all through the system.



Tools to measure your track and wheels:

When I started this investigation, the Aristo-craft Track & Wheel gauge ART-11906 came on the market. This is the first combination gauge available, so I relate the Aristo gauge to the standards below to see if it could be a workable tool. I use it's min/max feature to set my wheels.

(Yes it is missing the levelling vials, they are held in with hot glue and always fall out, they are pretty worthless anyway except at a gross level)

Originally I was very "hot" on this idea, but since you can buy a set of digital vernier calipers for about $20 at Harbor Freight, this is more of an exercise in convenience.

It is easy though for people that are not as anal as me ha ha!. I set everything to an exact value, rather than see if I am in range. This is my recommendation since once set they seem to be good forever. But the go/no go allows you some tolerance.

I suggest if you have one of these, measure it and adjust it to match the tolerances you are working with (hopefully following a standard!). Mine was not exact and needed a couple of whacks with a file to be right. There's also some variation in the gauges I have measured.

Kadee gauge:

I've also checked out the Kadee coupler height gauge, since it has a wheel and track gauge incorporated. It has 2 rectangular slots on one side to "measure" wheel gauge. It also has 2 prongs to measure track gauge. Honestly it's really not useful for these functions except to tell you if you are WAY out. Don't use it for these functions.

The track gauge is 1.762" - WAY OFF.

The wheel "gauge" is just plain goofy. It basically measures some range of the edges of the flanges, which is a terrible way to measure either back to back or gauge. It's not even worth giving a measurement here since it means nothing. The contour of the flange, the thickness of the very edge makes this really worthless.

Please, don't use this gauge except to determine if a wheel has moved GROSSLY out of gauge.


Here is the information on standards I have amassed:

Basic track gauge: 

Track gauge maximum (Aristo over all specifications, allows too wide track gauge in all cases of all known standards)

  • 1.799" Aristo gauge actual
  • 1.793" NMRA Proposed Standard S-1.2 (Standard)
  • 1.793" NMRA Proposed Standard S-1.3 (Hi-Rail)
  • 1.792" G1MRA Standard (target is 1.772")
  • 1.783" NEM 310 (target is 1.772")
  • 1.762" Kadee track gauge (one measurement only on this gauge)

(Note that NMRA Approved Standard S-1.3 did not specify max or min gauge, but gauge was specified as 1.772" absolute)
(Note here and in other measurements that the G1MRA original standard specified a target value with no tolerance +-)

Track gauge minimum (Aristo gauge under specification, "too tight", would "fail" some track that meets other minimum specs)

  • 1.772" Aristo gauge actual
  • 1.770" NMRA Proposed Standard S-1.3
  • 1.766" NMRA Proposed Standard S-1.2
  • 1.765" Kadee track gauge (one measurement only on this gauge)
  • 1.752" G1MRA Standard (target is 1.772")
  • NEM 310 no minimum specified (targes is 1.772")

(Note that NMRA Approved Standard S-1.3 did not specify max or min gauge, but gauge was specified as 1.772" absolute, therefore using this part of the gauge with no tolerance will meet the existing Hi-Rail gauge standard.)


Wheel back to back maximum (Aristo gauge perfect)

  • 1.755" Kadee gauge (Note: there are 2 notches, this is the measurement of the notches, when used to set an old Aristo "split axle" wheelset, it set the back to back at 1.587", EXTREMELY dependent on flange contour)
  • 1.594" Aristo gauge actual (Ted Doskaris measured 1.595")
  • 1.594" G1MRA Standard (Note target is 1.574")
  • 1.594" NMRA Standard maximum
  • 1.582" S-4.2 standard (standard 1:20.3 ) (Note target is 1.580")
  • 1.575 NMRA target
  • 1.574" S-4.3 standard (Hi-Rail) (Note target is 1.576")
  • 1.574" S-4.2 standard (Standard LS) (Note target is 1.576")

Wheel back to back minimum (Aristo too wide/restrictive in all cases, the true allowable minimums are smaller)

  • 1.591" Kadee gauge (Note: there are 2 notches, this is the measurement of the notches, when used to set an old Aristo "split axle" wheelset, it set the back to back at 1.587", EXTREMELY dependent on flange contour)
  • 1.578" S-4.3 standard (Hi-Rail) (Note target is 1.576")
  • 1.578" S-4.2 standard (Standard LS) (Note target is 1.576")
  • 1.575 NMRA target
  • 1.575" Aristo gauge actual
  • 1.574" G1MRA Standard (Note target is 1.574")
  • 1.570" NMRA standard minimum
  • 1.567" S-4.2 standard (Standard 1:20.3) (Note target is 1.580")


Flangeway width maximum (Aristo over spec) (at frog)

0.109" Aristo gauge
0.106" NMRA standard S-3.3
0.140" G1MRA (Note target is 0.120")

Flangeway width minimum (Aristo ?? spec) (at frog)

?? Aristo gauge
0.097" NMRA standard S-3.3
0.100" G1MRA (Note target is 0.120")

By calculating from the other specs, the flangeway width (on the stock rails) should be .097 min, .112 max for the guardrail width (on stock rails), but I would stick to 0.106 maximum.

2.69 to 3.33 (0.106" - 0.131") NMRA standard S-3.3 (only for flangeways on wing rails (frog), and only if for "G" only, designed to accomodate older G scale, and wheelsets with a back-to-back of 1.542", and you can no longer run #1 scale wheelsets).

Note: for the flangeways on the stock rails (the outer 2 rails of the switch) the spec is 2.69mm (0.106)

Flangeway depth (Aristo within spec)

3.04 mm (0.120") (approx) Aristo gauge for frog
3.00 mm minimum (0.118"), no max specified, NMRA standard S-3.3


Switchpoint spread maximum (??? someone help me out on this one)

Use standardard S1 to undedrstand definitions.

Use S3.2 for the measurements

43.18 mm Aristo gauge
41.5 - 41.63 NMRA standard S-3.2
(Note, this is for I gauge, the NMRA does not specify this for G gauge in the S-3.3 standard, I suspect this would be relaxed in the S-3.3 if it was specified)

Here is the link to see the standards:


My conclusion on using the Aristo gauge on track gauge: (check and modify it!)

Never use the maximum. It is way too wide. Use the minimum part of the gauge to check the track, but if it's "tight"  on the gauge, it could still be fine. Recommendation: file the Aristo gauge down in both places to meet the standard you want, I would suggest the Hi-Rail ones.

My conclusion on using the Aristo gauge on wheels: 

Never go to the maximum unless you want to use G1MRA standards, just make sure the wheels are plenty loose on the Minimum part of the gauge, and just start to "climb" over the Maximum part of the gauge. (This is the setting I use). This comes out to about 1.587"

My conclusion on using the Kadee gauge on wheels and track: (don't!)

It will only tell you if the wheelset is grossly out of gauge, and will set different back-to-back measurements based on flange thickness/contour. Use it as a rough check only.

Likewise, the track gauge is one measurement only. It is within specifications, but I again would use it only for a rough check.

My conclusion on using the Aristo gauge on flangeway width: (modify it)

Don't use it, use calipers to get to 0.106" or even less. This needs further research, but this sloppy spec is the #1 cause of wheel flanges hitting the frog.

Dave Goodsen recommends a flangeway width of 0.120" as a target.

I would file down the Aristo gauge to make it usable. 


My conclusion on using the Aristo gauge on flangeway depth:

Use it, it's only .002 deeper than the minimum spec, and you really do not want your flanges riding on the frog. File it down to meet the spec.


Turnout (switch) (points, if you are in England) Definitions

A switch is basically a construction that allows 2 or more tracks to combine to one, or vice versa.

so in most switches (except wyes and 3 way switches) one "path" is straight and one curved.

The outer 2 rails of the switch are called the "main rails", and you can call one the "straight" or "through" main rail, and the other the divergent rail. (avoid calling it the curved main rail, since there are curved turnouts also, and not all turnouts appear to have a curve)

On the 2 main rails, there are 2 smaller pieces of rail called "guard rails". The purpose of them is to keep the wheels on the proper course as the tracks "cross" each other at the frog. The back of the wheel flange is limited in travel by the guard rails.

The moving assembly inside that actually causes a train to take a certain route is called the "points" and those rails are also called point rails.

The part where the tracks "cross" each other is called the "frog".

The frog actually has several components, the point where the 2 rails in the middle come together, called the frog point (sometimes called the "frog heel"), and the 2 guard rails that sort of wrap around the point. These are called "wing rails". They function like the guard rails in controlling the wheels as they move through the frog.

Looking at the frog, you will see space where the wheels are not supported as they roll through. In real railroads, the angle of the frog is typically so slight that SOME portion of the wheel is always supported, or if not, the flange of the wheel will roll in the "flangeway" between the frog point and the wing rails.


Weather Underground PWS KCACARLS78