Wheel & Rail: Why Trains Track and Derail

Most derailments aren’t bad luck — they’re geometry. Slide the wheelset off-center and watch what a wheel actually does: it rolls on the railhead with a gap either side, then the flange touches, climbs the rail, and drops over the top. That last step is a derailment.

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← push toward left rail 0.000 in push toward right rail →

Start centered (a gap on both sides), then push toward a rail to make the flange climb.

Wheelset Rails Flange climbing Derailed

Simplified 2D geometry for insight, not contact mechanics. At rest the tread rides the railhead with a flangeway gap each side; the self-steering comes from the coned tread (rolling-radius difference ≈ 2 × conicity × shift). The climb follows the flange face angle and the Nadal L/V limit. Terms are explained in the glossary; confirm anything critical against the real NMRA/NEM/Proto standards.