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Proto-Fine Wheels
NMRA STANDARDS are the Primary System for interchange for standard gauge (4'-8 1/2") in each scale. The narrow gauge (n3) STANDARDS developed years later are built around closer to scale wheels and flangeways will be compatible with their standard gauge.
Scales large enough to use scale width wheels carry the designation 'Proto'. See related TRACK systems in RP-3, with their scale ratio, while smaller scales using over-scale (but narrower than STANDARD) width wheels are designated as 'Fine'.
| Name or Scale | Scale to Foot | K Check Gauge (max) |
B Back to Back (min) |
N Tire Width (min) |
D Flange Depth (max) |
|---|---|---|---|---|---|
| Proto 24 | .500" | 2.265" | 2.188" | .229" | .063" |
| (57.5mm) | (55.6mm) | (5.82mm) | (1.60mm) | ||
| Proto 24n3 | .500" | 1.411" | 1.334" | .229" | .063" |
| (35.8mm) | (33.9mm) | (5.82mm) | (1.60mm) | ||
| Proto 32 | .375" | 1.699" | 1.641" | .172" | .047" |
| (43.2mm) | (41.7mm) | (4.4mm) | (1.19mm) | ||
| Proto 32n3 | .375" | 1.058" | 1.000" | .172" | .047" |
| (26.9mm) | (25.4mm) | (4.4mm) | (1.19mm) | ||
| Proto 48 | .250" | 1.132" | 1.093" | .115" | .031" |
| (28.8mm) | (27.8mm) | (2.91mm) | (0.79mm) | ||
| S Fine | .188" | .839" | .800" | .108" | .030" |
| (21.3mm) | (20.3mm) | (2.74mm) | (0.76mm) | ||
| HO Fine | .138" | .613" | .581" | .086" | .026" |
| (15.6mm) | (14.8mm) | (2.2mm) | (0.66mm) | ||
| TT Fine | .100" | .441" | .415" | .071" | .022" |
| (11.2mm) | (10.5mm) | (1.80mm) | (0.56mm) | ||
| N Fine | .075" | .332" | .314" | .053" | .020" |
| (8.4mm) | (7.98mm) | (1.35mm) | (0.51mm) |
- NOTES:
-
- Notes 1 thru 6 of NMRA STANDARD S-4 also apply here.
- For Street Railway and Industrial Tram models where prototype wheels are narrower than 5 1/2 inches. Wheel tire Width Nmin may be reduced to twice the maximum Flangeway Fmax.
Scales larger than PROTO 24 (1/2"=1') are used primarily by live steamers who are well established in their respective sizes, each with its own formula for establishing wheel and track figures, thus they are not included here.