Model Railroad Track Spacing Calculator
Enter the radius, straight-track centers, and measurements from the limiting equipment in your roster. The result gives a custom TN-7 curve spacing alongside the applicable NMRA RP-7.2 table value.
Required curved track centers (in)2.48
- Outer track radius (in)
- 26.48
- Extra spacing on the curve (in)
- 0.48
- NMRA RP-7.2 reference minimum (in)
- 2.91
- Custom versus NMRA reference
- The custom result is narrower than the RP-7.2 reference by 0.425 in.
The custom calculation assumes concentric circular tracks without superelevation and uses a rectangular equipment envelope.
The RP-7.2 value is a separate reference lookup; when the radius falls between columns, it uses the next smaller listed radius.
How to use this calculator
- Choose inches or millimeters, then use that one unit for every length you enter.
- Choose the scale and equipment era used for the RP-7.2 reference lookup.
- Enter the inner track centerline radius and your intended tangent center spacing.
- Measure the vehicle with the longest end swing and the vehicle with the widest pivot spacing.
- Use the required curved centers and ease the added spacing back to the tangent alignment.
How curved track centers are calculated
Track center spacing is the distance from one track centerline to the next. On parallel straight track, the equipment stays parallel to the centerlines. On concentric curves, a long body moves inward between its pivots while a long end swings outward. Both motions reduce the clearance between trains, so a curve often needs wider centers than the adjacent tangent track.
The custom calculation follows the no-superelevation geometry in NMRA TN-7. Let R be the inner-track radius, T the tangent centers, C = T / 2 the equipment contact half-width, Pe the pivot spacing of the long-end vehicle, Le its longest pivot-to-end distance, and Pp the longest pivot spacing in the roster. With Me = Pe / 2, De = Me + Le, and Mp = Pp / 2:
Rsi = sqrt(R² − Me²) − C
Rei = sqrt((Rsi + 2C)² + De²)
S = sqrt((Rei + C)² + Mp²) − R
W = max(T, S)
S is the custom curved center spacing and W is the displayed requirement. The maximum matters: a curve never calls for centers narrower than the straight-track spacing already selected. The outer centerline radius is R + W; curve widening is W − T.
How the NMRA table is used
The RP-7.2 reference is separate from the custom result. It represents NMRA limiting equipment for the chosen scale and era, not necessarily the exact models on your layout. If the entered radius lies between printed columns, RP-7.2 directs you to use the next smaller listed radius; this calculator does not interpolate. If the radius is sharper than the smallest value in that scale's table, it reports no table recommendation rather than extrapolating.
What to measure and what can change the answer
For end swing, measure between the truck pivots, or between the outer rigid driver axles on a steam locomotive, then measure from the nearest of those pivots to the furthest fixed end projection. For inside swing, use the largest truck-pivot or bolster spacing found anywhere in the roster. Those can be different vehicles, and the same vehicle can supply both measurements. Small radii, long end overhang, and long bolster spacing all increase the custom requirement.
What this tool leaves out
This is a plan-view, rectangular-envelope calculation for concentric circular tracks without superelevation. Banking, coupler swing, diaphragms, articulated mechanisms, unusually wide loads, wheel and track play, transition geometry, and construction tolerance can all need more clearance. A spacing result also does not prove that equipment can negotiate the selected radius; follow the manufacturer's minimum-radius guidance and test the limiting equipment before fixing track or scenery.
Worked example
For HO classic equipment on a 24 in inner radius, use 2.00 in tangent centers, 2.63 in between the pivots of the longest-end vehicle, 2.98 in from its pivot to the furthest end, and 5.63 in for the longest bolster spacing. The TN-7 geometry gives custom curved centers of 2.481 in, so the required centers are 2.481 in. The outer-track radius is 26.481 in and the curve needs 0.481 in more spacing than the tangent.
The 24 in radius falls between the HO RP-7.2 columns for 26 5/8 in and 22 29/32 in. Following the next-smaller-radius rule uses the 22 29/32 in classic value: 2 29/32 in, or 2.906 in. The custom result is therefore 0.425 in narrower than this standards reference.
Common questions
How is model railroad track spacing measured?
Model railroad track spacing is measured centerline to centerline: from the middle of one track to the middle of the adjacent track. Measure the radius at the centerline of the inner track as well. Rail-to-rail gaps are not a dependable substitute because they vary by gauge and track construction.
Why do parallel tracks need more spacing on curves?
On a curve, the middle of a long car moves toward the curve center while the end corners move outward. Equipment on adjacent concentric tracks can therefore approach each other more closely than it does on straight track. Tighter curves and longer equipment make the effect larger.
Which rolling stock should I measure?
Measure the locomotive or car with the greatest fixed long-end projection for the end-swing inputs. Separately find the car or locomotive with the greatest rigid distance between truck pivots or equivalent rigid axles. Include the actual models you plan to run, not only their nominal prototype lengths.
What is the difference between bolster spacing and end overhang?
Bolster spacing is the distance between the two truck pivots and governs the inward movement of a long body on the curve. End overhang is the distance from a pivot to the furthest fixed point at that end and governs outward end swing. One vehicle can have the maximum of both dimensions.
Should I use the custom result or the NMRA table value?
Use the larger value unless you have a deliberate reason and physical testing to justify less clearance. The custom result reflects the equipment you measured, while RP-7.2 is a conservative published reference for its selected scale and era. Never use less than your intended tangent spacing.
Does superelevation change the required track centers?
Yes. Superelevation tilts the equipment inward and changes the clearance geometry. This calculator intentionally excludes it; use the full NMRA curved-track assistant or an equivalent method when the outer rail is elevated.