- Join the NMRA
- About the NMRA
- Education
- Resources
- Community
Search NMRA.org
RP-3 Proto-Fine Track
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' with their scale ratio, while smaller scales using over-scale (but narrower than STANDARD) width wheels are designated as 'Fine'.
See related WHEEL systems in RP-4
| Name or Scale | Scale or Ratio | G Track Gauge (min/max) |
C Check Gauge (min) |
S Span (max) | F Flangeway(max) | H Flange Clearance (min) |
P Switch Point Spread (max) (elec/mech) |
|---|---|---|---|---|---|---|---|
| Proto 24 | 1/2" | 2.354"*(59.8mm)/ | 2.265" | 2.184" | .104" | .063" | 2.239" (56.9mm)/ |
| 2.409" (61.2mm) | (57.5mm) | (55.5mm) | (2.64mm) | (1.60mm) | 2.244" (57.0mm) | ||
| Proto 24n3 | 1/2" | 1.500"*(38.1mm)/ | 1.411" | 1.330" | .104" | .063" | 1.385" (35.2mm)/ |
| 1.555" (39.5mm) | (35.8mm) | (33.7mm) | (2.64mm) | (1.60mm) | 1.390" (35.3mm) | ||
| Proto 32 | 3/8" | 1.766"*(44.8mm)/ | 1.699" | 1.638" | .079" | .047" | 1.678" (42.6)/ |
| 1.808" (45.9mm) | (43.2mm) | (41.6mm) | (2.00mm) | (1.19mm) | 1.683" (42.8) | ||
| Proto 32n3 | 3/8" | 1.125"* (28.6mm)/ | 1.058" | .997" | .079" | .047" | 1.037" (26.3)/ |
| 1.167 (29.6mm) | (26.9) | (25.3mm) | (2.00mm) | (1.19mm) | 1.042(26.5) | ||
| Proto 48 | 1/4" | 1.177"* (29.9mm)/ | 1.132" | 1.091" | .053" | .031" | 1.117" (28.4mm)/ |
| 1.205" (30.6mm) | (28.8mm) | (27.7mm) | (1.35mm) | (0.79mm) | 1.122" (28.5mm) | ||
| S Fine | 1:64 | .883"* (22.4mm)/ | .839" | .799" | .053" | .030" | .819" (20.8mm)/ |
| .905" (23.0mm) | (21.3mm) | (20.3mm) | (1.35mm) | (0.76mm) | .829" (21.1mm) | ||
| HO Fine | 1:87.1 | .649"* (16.5mm)/ | .613" | .580" | .040" | .026" | .600" (15.2mm)/ |
| .665 (16.9mm) | (15.6mm) | (14.7mm) | (1.02mm) | (0.66mm) | .605(15.4mm) | ||
| TT Fine | 1:120 | .471"* ( 12.0mm)/ | .441" | .414" | .033" | .022" | .430" (10.9mm)/ |
| .485" (12.3mm) | (11.2mm) | (10.5mm) | (0.84mm) | (0.56mm) | .435" (11.0mm) | ||
| N Fine | 1:160 | .353"* (8.97mm)/ | .332" | .313" | .025" | .020" | .322" (8.19mm)/ |
| .366" (9.30mm) | (8.44mm) | (7.95mm) | (0.64mm) | (0.51mm) | .327" (8.31mm) |
*Denotes Preferred Dimension
These 4 notes are primarily for O and On-3 gauges but may be useful in other smaller scales. (For dual gauge track)
- Lay the turnout for O scale to S-3 Standards.
- Add the On-3 only rail with an On-3 flangeway along it to S-3 Standards.
- To make guard rails along the common running rail work for On-3, move the On- 3 rail out by an amount equal to the difference in the flangeway widths that will maintain the On-3 check gauge.
- Floor all common flangeways to the maximum depth of flanges used by O scale.
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.