Overhang Clearance Calculator
Description
A rail car's trucks (the wheel assemblies) follow a curve exactly, but the car's body sitting on top of them is a rigid, straight shell that can't bend. That mismatch makes part of the car swing out of line on every curve. Two different parts move in two different directions. The center of the car, between its two trucks, swings toward the inside of the curve — it can clip a building, a tunnel portal, or anything else placed close to the inside of the curve. Meanwhile, each overhanging end of the car (anything sticking out past its nearest truck, like a locomotive's pilot or a car's end platform) swings toward the outside of the curve — it can instead clip something on the outside, or sideswipe a train on an adjacent parallel track.
This calculator works in whichever direction you need. The default mode works forward: measure your longest car or locomotive and enter your curve's radius, and it gives you both swing distances so you know how much clearance to leave on each side. The other two modes work backward: if you already know the most overhang you can allow — say, a tunnel portal or an adjacent track only leaves you a fixed gap — enter that limit instead, and the calculator gives you the minimum curve radius that stays within it.
It reports the car's own geometric swing only — add your own safety margin on top of these numbers.
Sources
- NEM 104 (MOROP), "Area Clearances for Narrow Gauge Railroads" — publishes the center overhang formula directly: E = R − √(R² − (A/2)²).
- NMRA Technical Note TN-7, "Curved Track Centerline and Obstacle Clearance Calculation Methodology" (Van S. Fehr, July 2017, NMRA Board-approved) — its equations for the radius to a car's outer long-end corner, with obstacle-clearance margin and superelevation set to zero, reduce to this calculator's end overhang formula.
- The two "minimum radius" modes solve each formula above algebraically for R — the same equations, rearranged, not a new or separately sourced formula.
Disclaimer: these are pure geometric swing amounts, not a recommended minimum clearance — add your own safety margin, and always check against your actual equipment and layout.