Rail transport modelling uses a variety of scales (ratio between the real world and the model) to ensure scale models look correct when placed next to each other. Model railway scales are standardized worldwide by many organizations and hobbyist groups.
Some of the scales are recognized globally, while others are less widespread and, in many cases, virtually unknown outside their circle of origin. Scales may be expressed as a numeric ratio (e.g. 1/87 or 1:87) or as letters defined in rail transport modelling standards (e.g. HO, OO, N, O, G, TT and Z.)
The majority of commercial model railway equipment manufacturers base their offerings on Normen Europäischer Modellbahnen (NEM) or National Model Railroad Association (NMRA) standards in most popular scales.
European Gauges
MOROP, the European model railway standards organisation, issues standards documents called NEMs . NEM010 defines the main model railway gauges, including narrow gauges.[2] Unusually, unlike the British model railway trade, this recognised narrow-gauge modelling from the outset. This may be because of Europe’s greater prototypical use of the larger narrow gauges for smaller branch lines.
NEM010 defines and names narrow gauges for all the supported scales although it takes a broad approach and groups the prototypes into ‘nominal size’ ranges or ‘Nenngröße’.
Names are of the form ‘H0e gauge’, comparable to 00-9, as ‘Narrow gauge in H0 scale’. Thus the scale and approximate prototype gauge are represented, with the model gauge used (9mm for H0e gauge; 6.5mm for H0f gauge) being implied.
| SCALE NAME | M | D | F | G or II | H | A | #1 OR I |
| ALTERNATE NAME | 7/8″ | 3/4″ | 3/5″ | 17/32″ | 1/2″ | 2/5″ | 3/8″ |
| SCALE RATIO 1: | 13.5 | 16 | 20.3 | 22.6 | 24 | 29 | 32 |
| INCHES/FOOT | 0.889 | 0.750 | 0.591 | 0.531 | 0.500 | 0.414 | 0.375 |
| MILLIMETERS/FOOT | 22.6 | 19.1 | 15.0 | 13.5 | 12.7 | 10.5 | 9.5 |
| TRACK GAUGE 45mm= | 23.9 | 28.3 | 36.0 | 40.0 | 42.5 | 51.4 | 56.7″ |
| REPRESENTING | 24.0 | 30.0 | 36.0 | 36.0 | 36.0 | 56.5 | 56.5″ |
| ERROR | 0.3% | 5.8% | 0.0% | -10% | -15% | 9.0% | -0.2% |
The scales used include the general European modelling range of Z, N, TT, H0, 0 and also the large model engineering gauges of I to X, including 3½”, 5″, 7¼” and 10¼” gauge. As 00 is a particularly British scale, it is not included within this pan European standard. However the predominantly US imperial-based S scale (1:64) does feature.