Practical tools for measured work
Car Overhang & Track Spacing on Curves
Find how far cars swing in and out on a curve and the minimum spacing between parallel tracks.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Centre-line radius of the inside track of the pair.
Length of the body, not over the couplers.
Distance between the two truck pivots.
Air gap you want between the closest points of two passing cars.
The diagram shows the worst case: the end of a car on the inside track beside the middle of a car on the outside track, with the spacing set to the minimum shown.
Minimum spacing at other radii
| Track radius | Minimum spacing |
|---|---|
| 18 in | 2.258 in |
| 24 in | 2.058 in |
| 30 in | 1.933 in |
| 36 in | 1.846 in |
| 48 in | 1.736 in |
How it works
Why the curve needs more room: a long car rides on two trucks that follow the curve, but the body is straight. The middle of the body cuts inside the track centre line and the ends swing outside it. On parallel tracks the swing of one car adds to the swing of the other.
Method: both truck pivots sit on the track radius R, so the chord between them lies at a = √(R² − (t ÷ 2)²) from the curve centre, where t is the truck spacing. The inside edge of the body is at a − w ÷ 2 and its outer end corner at √((L ÷ 2)² + (a + w ÷ 2)²), where L is the body length and w the width. The minimum spacing puts the outer swing of one car, plus your clearance, at the inner swing of the other.
Measuring: measure body length and width without couplers and steps, and measure truck spacing between the pivot centres. Test with the longest and widest car you run.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Measurement unit
- Inches
- Track radius
- 24
- Car body length
- 11.7
- Truck centre spacing
- 8.2
- Car body width
- 1.14
- Clearance between cars
- 0.25
Primary result: Minimum track spacing on the curve: 2.058 in.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This assumes two identical rigid cars on concentric circular tracks and uses the worst alignment, so the result is conservative. It does not include coupler swing, body roll, wide loads, overhanging details, superelevation or easements. Compare the result with the NMRA recommended practice for track spacing and test with real cars before fixing track centres.
Frequently asked questions
Why does spacing grow as the radius shrinks?
Tighter curves make the body swing farther from the track centre line. The table shows how the required spacing changes at larger radii so you can see what a wider curve saves.
Does it work for different car types?
Enter the dimensions of the car that overhangs the most. If your longest car is on the inside track and a wide car passes it, add extra clearance.
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