See also: Rail transport modelling scales
 | It has been suggested that this article be merged into Rail transport modelling scales. (Discuss) Proposed since July 2024. |
This page lists the most relevant model railway scale standards in the world. Most standards are regional, but some have followers in other parts of the world outside their native region, most notably NEM and NMRA. While the most significant standardised dimension of a model railway scale is the gauge, a typical scale standard covers many more aspects of model railways and defines scale-specific dimensions for items like catenary, rolling stock wheels, loading gauge, curve radii and grades for slopes, for instance.
Scale standards
NEM
MOROP (the European federation of national model railway associations) is a European organisation which publishes NEM-standards. NEM-standards are used by model railway industry and hobbyists in Europe. The standards are published in French and German and both versions have an official status. Unofficial translations in English from third parties exist for certain NEM-standard sheets.
Model railway scales and gauges are standardized in NEM 010, which covers several gauges for each scale. Narrow gauges are indicated by an additional letter added after the base scale as follows:
- no letter = standard gauge (1,250–1,700 mm or 49.2–66.9 in)
- m = metre gauge (850–1,250 mm or 33.5–49.2 in)
- e = narrow gauge (650–850 mm or 25.6–33.5 in)
- i = industrial (400–650 mm or 15.7–25.6 in)
- p = park railway (300–400 mm or 11.8–15.7 in)
For instance, a metre-gauge model railway in H0-scale is designated H0m. In German text the letter "f" (for Feldbahn) is sometimes used instead of "i". The letter "e" represents the French word for "narrow", étroit. NEM gauges are arranged conveniently to use the normal gauge of smaller scales as narrow gauges for a certain scale. For instance, H0m gauge is the same as the TT-scale normal gauge, H0e same as the N-scale normal gauge and H0i same as the Z-scale normal gauge.
For H0 and 0 scales, NEM uses the number zero, and NMRA uses letter "O" (HO instead of H0).
| Scale | Ratio | Standard gauge | m | e | i | p | Notes |
|---|---|---|---|---|---|---|---|
| Z | 1:220 | 6.5 mm (0.256 in) | 4.5 mm | – | – | – | Standard gauge based on the Märklin factory standards. |
| N | 1:160 | 9 mm (0.354 in) | 6.5 mm | 4.5 mm | – | – | Standard gauge based on the Arnold factory standards. |
| TT | 1:120 | 12 mm (0.472 in) | 9 mm | 6.5 mm | 4.5 mm | – | – |
| H0 | 1:87 | 16.5 mm (0.65 in) | 12 mm | 9 mm | 6.5 mm | 4.5 mm | "Half zero" |
| S | 1:64 | 22.5 mm (0.886 in) | 16.5 mm | 12 mm | 9 mm | 6.5 mm | – |
| 0 | 1:45 | 32 mm (1.26 in) | 22.5 mm | 16.5 mm | 12 mm | 9 mm | |
| 1 | 1:32 | 45 mm (1.772 in) | 32 mm | 22.5 mm | 16.5 mm | 12 mm | – |
| II | 1:22.5 | 64 mm (2+1⁄2 in) | 45 mm | 32 mm | 22.5 mm | 16.5 mm | Standard gauge known as Gauge 3 in the UK and known as Gauge 2+1⁄2 on the Märklin factory standards from 1891. |
| III | 1:16 | 89 mm (3+1⁄2 in) | 63.5 mm | 45 mm | 32 mm | 22.5 mm | |
| V | 1:11 | 127 mm (5 in) | 89 mm | 63.5 mm | 45 mm | 32 mm | Common gauge for Live Steam |
| VII | 1:8 | 184 mm (7+1⁄4 in) | 127 mm | 89 mm | 63.5 mm | 45 mm | Common gauge for Live Steam |
| X | 1:5.5 | 260 mm (10+1⁄4 in) | 184 mm | 127 mm | 89 mm | 63.5 mm | – |
NMRA
The NMRA (National Model Railroad Association) standardized the first model railway scales in the 1940s. NMRA standards are used widely in North America and by certain special interest groups all over the world. To some extent NMRA and NEM standards are compatible, but in many areas, the two standards specify certain model railway details in somewhat incompatible ways for the same scale.
There are two NMRA standard sheets where the scales have been defined. NMRA standard S-1.2 covers the popular model railway scales and S-1.3 defines scales with deep flanges for model railways with very sharp curves or other garden railway specific design features.
In certain NMRA scales an alternative designation is sometimes used corresponding the length of one prototype foot in scale either in millimetres or in inches. For instance, 3.5 mm scale is the same as HO. For HO and O -scales, NMRA uses the letter "O" whereas NEM uses the number zero (H0 instead of HO).
The NMRA published alternative, more accurate and realistic standards for track and wheels sheet in S-1.1 These model railway standards are based on the full size prototype standards and the scale model operational reliability is therefore reduced in comparison to the models conforming to the normal NMRA standards. Proto and finescale rails and wheels are generally not compatible with the normal scale model railway material with the same scale ratio.
Proto scale was originally developed by the Model Railway Study Group in Great Britain in 1966 and later adopted into NMRA standards with modifications necessary for the North American prototype railway standards. Proto scale reproduces faithfully the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association.
Finescale reproduces the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association with minor compromises for performance and manufacturability.
NMRA popular railway scales
| Scale | Ratio | Model gauge | Notes |
|---|---|---|---|
| Z | 1:220 | 6.5 mm (0.256 in) | NMRA does not give any other dimensions for Z scale apart from the gauge The s.g. is set nominally to gauge=6.5 mm; more exact to 1:220 would be 6.52 mm (0.257 in) |
| Nn2 | 1:160 | 0.177 in (4.5 mm) 4.5 mm (0.177 in) |
narrow gauge |
| Nn3 | 1:160 | 0.256 in (6.5 mm) 6.5 mm (0.256 in) |
narrow gauge |
| N | 1:160 | 0.353 in (8.97 mm) | standard gauge |
| TT | 1:120 | 0.470 in (11.94 mm) 0.472 in (12 mm) 12 mm (0.472 in) |
standard gauge |
| HOn2 or 3.5 mm | 1:87.1 | 7 mm (0.276 in) | narrow gauge |
| HOn30 or 3.5 mm | 1:87.1 | 9 mm (0.354 in) (0.353 in) | narrow gauge – N-scale track can be used |
| HOn3 or 3.5 mm | 1:87.1 | 10.5 mm (0.413 in) | narrow gauge |
| HOm or 3.5 mm | 1:87.1 | 12 mm (0.472 in) | narrow gauge – TT-scale track can be used |
| HO or 3.5 mm | 1:87.1 | 0.65 in (16.5 mm) | standard gauge |
| OO or 4 mm | 1:76.2 | 16.5 mm (0.65 in) | standard gauge |
| Sn3 or 3/16 in | 1:64 | 0.563 in (14.3 mm) | narrow gauge |
| S or 3/16 in | 1:64 | 0.883 in (22.43 mm) | standard gauge |
| On2 or 1/4 in | 1:48 | 12.7 mm (0.5 in) | narrow gauge |
| On30 or 1/4 in | 1:48 | HO-track | narrow gauge |
| On3 or 1/4 in | 1:48 | 0.75 in (19.05 mm) 19.4 mm (0.764 in) (?) |
narrow gauge |
| O or 1/4 in | 1:48 | 1.25 in (31.75 mm) | 1.177 in (29.9 mm) is true standard gauge |
| No. 1n2 or 3/8 in | 1:32 | 0.75 in (19.05 mm) | narrow gauge (same as On3 gauge) |
| No. 1n3 or 3/8 in | 1:32 | 1.125 in (28.6 mm) | narrow gauge |
| No. 1 or 3/8 in | 1:32 | 1.766 in (44.85 mm) | standard gauge |
| Fn3 or 15 mm | 1:20.32 | No. 1-track | narrow gauge |
| F or 15 mm | 1:20.32 | 2.781 in (70.69 mm) | Identical to Proto 20.32 except the wheel flange depth |
| 3/4 in | 1:16 | 3+1⁄2 in (89 mm) | standard gauge |
| 1 in | 1:12 | 4+3⁄4 in (121 mm) | Common gauge for Live Steam |
Note: to interpret the number in the left-hand column, these examples illustrate:
- 3.5 mm scale (HO): 3.5 mm scale measurement = 1 foot (304.8 mm) prototype. The ratio is therefore 1:87.08571, usually reported as 1:87.
- 1 in scale: 1 in scale measurement = 1 foot prototype, the ratio is reported as 1:12.
NMRA deep flange scales
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| SHR or 3/16 in | 1:64 | 0.865 in (21.97 mm) vs. 0.865 in (22 mm) |
– |
| O27 | – | – | Same as OHR but models 10% smaller on the same track gauge |
| OHR or 1/4 in | 1:48 | 1.25 in (31.75 mm) | – |
| G or 3/8 in | 1:32 | 1.772 in (45 mm) | – |
| G | 1:29 | 1.772 in (45 mm) | – |
| G | 1:24 | 1.772 in (45 mm) | – |
| G | 1:22.5 | 1.772 in (45 mm) | – |
| G | 1:20.3 | 1.772 in (45 mm) | – |
NMRA proto scales
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| Proto:20.32 | 1:20.32 | 70.69 mm (2.781 in) | NMRA |
| Proto:20.32n3 | 1:20.32 | 1.772 in (45 mm) | – |
| Proto:32 | 1:32 | 1.766 in (44.85 mm) | – |
| Proto:32n3 | 1:32 | 1.125 in (28.6 mm) | – |
| Proto:48w5 | 1:48 | 1.25 in (31.75 mm) | Russian prototypes |
| Proto:48 | 1:48 | 1.177 in (29.9 mm) | – |
| Proto:48n3 | 1:48 | 0.75 in (19.05 mm) | – |
| Proto:64 | 1:64 | 0.883 in (22.43 mm) | – |
| Proto:64n3 | 1:64 | 0.563 in (14.3 mm) | – |
| Proto:87 | 1:87.1 | 0.65 in (16.5 mm) | – |
| Proto:87n3 | 1:87.1 | 0.413 in (10.5 mm) | – |
NMRA finescale
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| Fine:HO | 1:87.1 | 0.65 in (16.5 mm) | – |
| Fine:HOn3 | 1:87.1 | 0.413 in (10.5 mm) | – |
| Fine:TT | 1:120 | 0.470 in (11.94 mm) 0.472 in (12 mm) |
– |
| Fine:N | 1:160 | 0.353 in (8.97 mm) | – |
| Fine:Nn3 | 1:160 | 0.25 in (6.35 mm) | – |
British
The main railways in Great Britain use the international standard gauge of 4 ft 8+1⁄2 in (1,435 mm) but the loading gauge is narrower and lower than in the rest of Europe with the same standard gauge. This is one of the main reasons why the country has traditionally used its own distinctive model railway scales which can rarely be found outside the British Isles.
When H0 scale was being introduced, the motors available were too large to fit in scale-sized bodies and so as a compromise the scale was increased from 3.5 mm to 4 mm to the foot, but the gauge was not changed so other elements could be shared. For 00 therefore the track is about 12.5% narrower than it should be for the scale used. EM and P4 standards correct this anomaly by adopting a wider track gauge.
The globally more-widespread international NEM and NMRA scale standards are relatively rare in Great Britain and used almost exclusively by those modelling foreign prototypes.
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| 000 or 2 mm | 1:152 | 9.42 mm (0.371 in) | An early predecessor of small scales like N. Developed before World War II and became somewhat popular in the 1950s. No commercial products available. Today The 2mm Scale Association is the force behind the scale and 2 mm scale has become a finescale alternative to the British N-scale. |
| N | 1:148 | 9 mm (0.354 in) | A British adaptation of N-scale for modelling British prototypes with a smaller loading gauge. Has a track gauge error of approximately −7%. Hobbyists who model European or American prototypes in Britain use the standard N-scale with the scale ratio 1:160. |
| TT:120 or 2.54 mm | 1:120 | 12 mm (0.472 in) | Track was introduced by Peco in 2022, closely followed by a range of locomotives, rolling stock and track from Hornby. This has the accurate scale and gauge combination of 2.54 mm : 1 ft on 12 mm track, as has been used by European and American hobbyists, who refer to this as TT. |
| TT3 or 3 mm | 1:102 | 12 mm (0.472 in) | Introduced by Triang in 1957 as a British adaptation of the American TT scale. Later Triang dropped this scale in favour of N scale and today there is no commercial following. Has a track gauge error of approximately −15% (the rolling stock superstructures are too large for its gauge). Those models can drive on straight and curved sections of German BTTB Standard Gleis except crossings and switches. |
| H0 or 3.5 mm | 1:87 | 16.5 mm (0.65 in) | H0 scale was introduced in Britain in the 1920s, and although it stayed as the most common worldwide modelling scale, in Britain H0 has little commercial availability and is generally only used to model the British prototype by a small number of modellers. |
| 00 or 4 mm | 1:76 | 16.5 mm (0.65 in) | The most popular railway modelling scale in Britain. The correct track gauge at the scale of 4 mm per foot should be approximately 18.8 mm. At 16.5 mm, it has a track gauge error of approximately −12.4%. |
| EM or 4 mm | 1:76 | 18.2 mm (0.717 in) | This gauge is represented by the EM Society (in full, Eighteen Millimetre Society). 00 track (16.5 mm) is the wrong gauge for 1:76 scale, but use of an 18.2 mm (0.717 in) gauge track is accepted as the most popular compromise towards scale dimensions without having to make significant modifications to ready-to-run models. Has a track gauge error of approximately −3.5%. |
| P4/S4 or 4 mm | 1:76.2 | 18.83 mm (0.741 in) | Uses a track gauge of 18.83 mm which represents an exact scaling down of the prototype at 4 mm to 1 ft scale. P4 contains an allowance for the tighter curves found on model railways in the wheel back-to-back and related dimensions. S4 removes this allowance, for a dead-scale representation of all trackwork dimensions. Both standards are now maintained by the Scalefour Society and are now applied to other prototypical track gauges (such as Irish broad gauge). |
| S | 1:64 | 22.45 mm (0.884 in) | S-scale is the same proportion in NEM, NMRA and the UK. S Scale Model Railway Society is the custodian of British S-scale. |
| O14 | 1:43.5 | 14 mm (0.551 in) | For accurately representing 2 ft narrow gauge in 7 mm scale. |
| 0 or 7 mm | 1:43.5 | 32 mm (1.26 in) | Three sub-standards: coarse, unified and fine. Has a track gauge error of approximately −3%. |
| ScaleSeven (S7) 7 mm | 1:43.5 | 33 mm (1.3 in) | A finescale represented by The ScaleSeven group. |
Japanese
While there are Japanese model railway manufacturers that export their products to other parts of the world and follow the scale standards of the export destinations, in Japan there are several domestic scales that are popular in the country but virtually unknown elsewhere. International NEM and NMRA scales are also used by some Japanese modellers. The main reason for the domestic scales different from international standards is the smaller prototype loading gauge and unusual gauges of Japanese railways: 600 mm (1 ft 11+5⁄8 in), 762 mm (2 ft 6 in) and 1,067 mm (3 ft 6 in) are used, along with standard gauge of 1,435 mm (4 ft 8+1⁄2 in).
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| T | 1:450 1:480 |
3 mm (0.118 in) | Launched at 2006 Tokyo Toy Show by Eishindo Co. |
| ZZ | 1:300 | 4.8 mm (0.189 in) | Introduced 2005 by Bandai |
| Z or ZJ | 1:220 | 6.5 mm (0.256 in) | |
| N | 1:150 1:160 |
9 mm (0.354 in) | The most popular scale in Japan. For models of Shinkansen high speed trains and other systems using standard gauge track, the international N scale standard ratio of 1:160 is commonly used. |
| TT9 | 1:120 | 9 mm (0.354 in) | Used also in New Zealand. |
| HOn2+1⁄2 | 1:87 | 9 mm (0.354 in) | Used for 2 ft 6 in (762 mm) narrow gauge. |
| 13 mm | 1:80 | 13 mm (0.512 in) | Correct for 1,067 mm (3 ft 6 in) Japanese narrow gauge. |
| #16 | 1:80 | 16.5 mm (0.65 in) | Used for ready-to-run models of 1,067 mm (3 ft 6 in) prototypes. |
| HO | 1:87 | 16.5 mm (0.65 in) | Used for models of Shinkansen high speed trains and other systems using standard gauge track. |
| HOj or HOn3-1/2 | 1:87 | 12 mm (0.472 in) | Correct for 1,067 mm (3 ft 6 in) Japanese narrow gauge. |
| Sn3+1⁄2 | 1:64 | 16.5 mm (0.65 in) | Used for ready-to-run models. |
| - | 1:50 | 20.5 mm (0.807 in) | Static models mostly. |
| OJ | 1:45 | 24 mm (0.945 in) | Correct for 1,067 mm (3 ft 6 in) Japanese narrow gauge. |
| O | 1:45 (1:40-1:42) |
32 mm (1.26 in) | 1:40-1:42 used about 1930s. |
| 35 mm | 1:30 | 35 mm (1.378 in) | Used for 1,067 mm (3 ft 6 in) Japanese narrow gauge. |
| # I or 45 mm | 1:30 | 45 mm (1.772 in) |
Lego trains
Lego trains use a fixed nominal gauge of 37.5 mm (1+15⁄32 in), based on 5-stud track centerlines gauge.
The 37.5 mm length is not derived by a certain scale ratio. While HO scale is a 1:87 scale (3.5 mm to 1 foot), resulting in a 16.5 mm (0.65 in) gauge from real life prototype 1,435 mm (4 ft 8+1⁄2 in) standard gauge standard gauge.
Conversely, modeling standard gauge in Lego trains would yield a scaling of (37.5:1435 =) 1:38.3.
| Scale | Ratio | Standard gauge | m | e | i | p | Notes |
|---|---|---|---|---|---|---|---|
| Lego Trains | varies | 1:38.3 ⇆ 37.5 mm (1+15⁄32 in) | Not by fixed scale ratio |
Live steam
Live steam model railways are not standardized systematically by any single standardization body. There are, however, certain scales and gauges which have become de facto standards and in some cases correspond to either NEM or NMRA standard scales. One example is the "IBLS" (International Brotherhood of Live Steamers), an informal organization which has published standards for some of the gauges. Many clubs have their own standards, which also may vary slightly from country to country. Hornby Railways have pioneered commercial model live steam in 00 (1:76 scale on 16.5 mm gauge), the existing models are heated using a controllable electric current through the two running rails and have the steam pressure chamber in the model tender.
In addition to these scales, the United Kingdom has, over the last forty years, fathered a scale that is based on the predominant British narrow track gauge of 2 ft (610 mm). Using 32 mm (1.26 in) - 0 gauge - track, there is an extensive range of 16 mm to the foot scale live-steam and other types of locomotives, rolling stock and accessories. Many of these models are dual gauge, and can be converted to run on 45 mm (1.772 in) track (gauge 1), and radio control is common. Locomotives in this scale are generally large and "chunky", and can range from the tiny 0-4-0 seen on Welsh slate quarry lines all the way up to the very largest found in the UK, such as the ex-ACR NG/G16 Beyer-Garratt locomotives, seen running on the Welsh Highland Railway in North Wales. The hobby is supported by a number of 16 mm live steam and electric traction builders, dominated by the likes of Roundhouse Engineering and Accucraft UK.
| Scale | Ratio | Standard gauge | Notes |
|---|---|---|---|
| 00 | 1:76 | 16.5 mm (0.65 in) | Hornby produced. Generally regarded as the smallest scale for live steam. Discontinued in 2011 due to poor sales and reliability of the units. The operating instructions from Hornby were incorrect and led to many locomotives being broken after derailing and crashing to the floor. There is still much interest and lively sales of secondhand sets. The 00 Live Steam Club also promotes the product. |
| O | 1:45 | 32 mm (1.26 in) | Popularly used for the small scale live steam. |
| No.1 | 1:32 | 45 mm (1.772 in) | Popularly used for the small scale live steam. Corresponds to NEM 1 or NMRA No. 1. |
| No.3 | 1:22.6 | 2+1⁄2 in (64 mm) | The smallest scale able to pull real passengers. Was one of the first popular live steam gauges, developed in England in the early 1900s. In terms of model railway operation, gauge 3 is the largest (standard gauge) scenic railway modelling scale, using a scale of 13.5 mm to the foot. The Gauge '3' Society represents this aspect of 2+1⁄2-inch gauge railway modelling with both electric and live steam operation. Gauge '3' corresponds to NEM II scale, also known as "Spur II" in Germany.
The National 2.5 in Gauge Association continues to support live steam passenger hauling in 2.5-inch gauge using MES tracks. They use a "scale" appropriate to the original prototype modelling both standard and narrow gauge locomotives to run on 2.5-inch track. |
| - | 1:16 | 3+1⁄2 in (89 mm) | A worldwide garden railroad scale. Corresponds to NEM III and NMRA 3⁄4 inch. |
| - | 1:12 | 4+3⁄4 in (121 mm) | North America specific scale corresponding to NMRA 1-inch scale. 1:12 is one of the most popular backyard railway scales. |
| - | 1:11 | 5 in (127 mm) | Used outside North America. Corresponds to NEM V. One of the most popular garden railway scales. Common gauge for live steam |
| - | 1:8 | 7+1⁄4 in (184 mm) | Used in North-Eastern US, Canada, Europe and other parts of the world. Corresponds to NEM VII. Common gauge for live steam |
| - | 1:8 | 7+1⁄2 in (190.5 mm) | Used in the Western parts of the US. Common gauge for live steam |
| - | 1:7.5 | 1.6 in=1 ft. Used in the US, often finer-scale. Uses 7+1⁄2 in (190.5 mm) gauge. Common gauge for live steam |
Static model
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| 41 mm | 1:35 | 41 mm (1.614 in) | 41 mm is used by several static model manufacturers, often for models of military subjects such as railway guns as 1:35 is a popular scale for military vehicle modelling. 41 mm was introduced in the 2000s.
|
Historical
There have been many short-lived and often promising model railway scales which are very much defunct nowadays. Quite often these were backed by only the company that created a new scale in the first place.
| Scale | Ratio | Gauge | Notes |
|---|---|---|---|
| K | 1:180 | 8 mm (0.315 in) | Introduced 1948 at the Hannover Fair in Germany by Walter M. Kersting. |
| Wesa | 1:110 | 13 mm (0.512 in) | Produced by WESA AG in Switzerland Between 1945 and 1950, used AC power. |
| Wesa | 1:100 | 13 mm (0.512 in) | Produced by WESA AG in Switzerland Between 1950 and 1966, used DC power. |
| OOO | – | 22 mm (0.866 in) | Introduced in 1902 by Schoenner in Germany. |
| OO | – | 23 mm (0.906 in) | Introduced in 1908 by Märklin and sold under name "Liliput-Eisenbahn" until 1932. |
| Z0 | 1:60 | 24 mm (0.945 in) | Z0 or "Zwischen Null" (between 0) was in use in the 1940s and 1950s by several model railway manufacturers in Germany. Standardized in East German NORMAT model railway standard collection. Z0 was originally introduced in Czechoslovakia in 1938. |
| 35 mm | 1:30 | 35 mm (1.378 in) | 35 mm was in use in the 1930s and 1940s by several model railway manufacturers in Japan. 35 mm was introduced in the 1930s. Late 1940s 35 mm was replaced by O gauge. |
| No. 2 | 1:27 | 2 in (50.8 mm) | English scale, 7⁄16 inch to 1 foot. Commercially used about 1900 |
See also
- National Model Railroad Association
- Normen Europäischer Modelleisenbahnen
- Rail transport modelling scales
- Rail transport modelling
- List of narrow gauge model railway scales
References
- "Normen Europäischer Modellbahnen Maßstäbe, Nenngrößen, Spurweiten" (PDF) (in German). MOROP. 2011. Retrieved 2014-05-15.
- NMRA 70.64mm
- NMRA 70.69mm
- "Scale and Gauge".
- http://l-gauge.org/index.php?title=Track_Geometry
- "Lego Specifications". Orionrobots.co.uk. 26 February 2011. Archived from the original on 23 August 2011. Retrieved 30 August 2022.
- Lego § Design; File:Lego dimensions.svg (stud dimensions)
- "Military Scale Model".
- CMK
- Dragon-models
- "Trumpeter". Archived from the original on 2019-12-06. Retrieved 2013-01-12.
External links
- "About Gauge" guide from Lionel discussing O versus O27 gauge
- "More About Gauge" guide from Lionel discussing gauges other than O