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A Quick Primer on North American Model Railroading Gauge and Scales

Generally North American, European, Great Britain, Japan, and Australia railroad modeling communities each maintain their own standards.  For now let’s focus on the Standards applicable to the North American community.

Gauge is the distance between the rails the train runs on.  Basically, stability increases with gauge. Narrow gauges are cheaper and faster to build than Broad gauges, while the Broad gauges are able to handle heavier and faster traffic.   In reality there are many different regional gauges in use around the world, but for practicality the North American modeling community recognizes 5 gauges:

  • Universal Standard Gauge (1435.1 mm)

  • 3 1/2 ft/Meter (1066.8 mm – 1000.0 mm)

  • 3 ft (914.4 mm), 2 1/2 ft (914.0 mm)

  • 2 1/2 ft (762.0 mm)

  • 2 ft (609.6 mm).

Scale is the ratio of the model dimension to the prototype.  The National Model Railroad Association (NMRA) standards use letters to differential the scales between 1:20.3 through 1:220 for the North America model railroading community.  

NMRA S-1.2 General Standard Scales

  • F indicates a 1:20.32 scale to the prototype

  • H indicates a 1:24 scale to the prototype

  • O indicates a 1:48 scale to the prototype

  • S indicates a 1:64 scale to the prototype

  • HO indicates a 1:87.1 scale to the prototype

  • TT indicates a 1:160 scale to the prototype

  • N indicates a 1:220 scale to the prototype

Since there is effectively 5 possible prototype gauges, narrow gauges are designated with a n42, n3, n30, or n2 subscript.  For example; O refers to a 1:48 scale model with a Universal Standard Gauge prototype gauge, while On30 refers to a 1:48 scale model with a 3 ft prototype gauge. 

  • No subscript indicates a Universal Standard Gauge Prototype gauge

  • n42 indicated 3 1/2 ft or Meter Prototype gauge

  • n3 indicated 3 ft Prototype gauge

  • n30 indicated 2 1/2 ft Prototype gauge

  • n2 indicated 2 ft Prototype gauge

Unless you are modeling a specific rail line or era you’re probably going to stick with a Universal Standard Gauge prototype.

Another scale variant is the minimum curve. The minimum radius to navigate a curve is determined by the trucks.  O-27 refers to O scale equipment that requires a minimum radium of 27 inches to navigate a curve without binding or derailing.

Additionally you may have heard about Standard Gauge, #1 Gauge or G Gauge/Scale.

Not to be confused with Universal Standard Gauge, which itself is not universal, is Lionel’s Standard Gauge. Lionel’s Standard Gauge was the first formal model railroading standard (patented 1906). Standard Gauge uses 2 1/4 inch (53.975 mm) gauge track along with a 1:26.58 scale. Similarly, Wide gauge used a 2 1/4 inch gauge with a slightly smaller 1:28.25 scale to avoid the Lionel patent.

#1, #2, #3, #4, #5, and #6 Gauge or Gauge 1-6 were first introduced by Marklin at the 1891 Leipzig Toy Fair and established as community model railroading standards in the August 1909 issue of Model Railways and Locomotive magazine. The scales #0 (today O) and H0 (today HO) derive from these standards. Since most of the Meter Prototype track wasn’t laid until the rebuilding of Europe following WWII, the Gauge 1-6 uses a Standard Gauge prototype.  #1 Gauge is a 1;32 scale model on a 45 mm gauge, #2 Gauge is 1:29 scale on 50.4 mm gauge, and #3 Gauge is 1:22.5 on 63.5 gauge. Remember that for later.

After WWI the larger gauges were rendered commercially obsolete by the Depression and the miniaturization of electric motors to the point that electrically powered O and HO scale models became possible. These smaller scales were also cheaper to manufacture and required less room to operate.

Which brings us to G Gauge. G is not an actual standard, it is a marketing term. Instead of G referring to a constant scale, G refers to any model railroad equipment that runs on the 45 mm gauge, regardless of scale.  Many manufacturers have multiple product lines with different scales marketed under the G name so scale compatibility needs to be researched if that sort of thing bothers you.

The best explanation (to me) of G that I have found is a very well written article from Rick Henderson from concerning what he called the 5 faces (or was it facets) of G. Unfortunately it appears the site has been abandoned as of 2015 and the links to past article return a 404 Not Found Error. What follows is a brief summary:

The 5 faces of G refers to the 5 most common commercially suported scales used on 45mm gauge track, and a theory about how G became a common gauge reference instead of a common scale reference.

The first face of G is the original G scale was LGB’s 1:22.5 first introduced in the 1960’s. As a previous manufacturer of both #1 Gauge and #3 Gauge, LGB would of had access to the equipment and plans for producing 1:22.5 scale stock and 45 mm track. Combine the two and you get the correct 45 mm scale for a meter gauge prototype.

Face number 2? As other manufactures began to note the popularity of LGB’s G Scale, eventually #1 Gauge is revived which just happens to be the correct 45 mm scale for a Universal Standard gauge prototype.  Rather than attempt to market another gauge, #1 Gauge manufacturers just marketed thier products as G to take advantage of LGB’s G product popularity.
Face number 3? As popularity increases the F scale (1:20.3) community realizes that a 1:20.3 scale using a 45 mm gauge is actually the NMRA 1936 Fn3 standard, or equivalent to a 3 FT Prototype gauge.
Face number 4? 1:24 is a popular toy scale. A 3 1/2 FT Prototype gauge results in 45 mm.  All of a sudden the 1:24 scale, 45 mm gauge community is the envy of all other due to their compatibility with the 1:24 scale toy market.

For the 5th scale you’re probably thinking right now, a 2 FT Prototype gauge results in a scale of 1:13.7 at a 45 mm gauge and your right, but that’s not the 5th face of G.  Maybe it was due to the large size, or the fact that there was no previous correlation to an existing standard, but the 2 FT Prototype gauge never gained a G Gauge following.

Earlier I asked you to remember the #1, #2, and #3 Gauge scales.  Those that paid attention may have noticed we have already covered 1:32 and 1:22.5 scales.  As the 1:29 scale existed previously as #2 Gauge, the manufacturing history was there to exploit and thus 1:29 is the 5th Face of G.  The theory goes that at some point the 1:32 scale G community community saw the 1:24 scale toy offerings and tried to incorporate the two. For some the difference is scale was just too much and the 1:29 scale was offered as a compromise, splitting the difference between the two scales.

With the resurgence of the Large Scale model trains came the resurgence of Lionel Standard Gauge in the 1970s.

Not to be outdone, the Gauge 3 Society has been dedicated to the 1:22.5 scale, 45 mm gauge cause since 1990.

Source References:,,,,,,,,,,,,,,,, http://www.peco‐,, http://www.o‐,, http://www.tiny‐,,, http://www.fremo‐,,,,,,‐started/popular‐scales/

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