Curvature change on grade
Work in progress: This section is still a work in progress — content is being added and revised continuously.
What is checked? Does the curvature change within a ramp (straight→curve, radius change, reverse curve)?
Why does it matter? Downhill the train pushes: the close-coupling cams are compressed and cannot swing out in the following curve — the heavy locomotive shoves the first car off the track. It happens reproducibly and is almost always missed at the planning stage because it looks harmless in a 2D plan.
How to fix it: Keep one constant radius across the whole ramp — or move the curvature change onto the level section before/after the ramp. This is exactly why helixes are built with a single radius throughout.
Numbers & defaults: checked from 1.5 % grade (editable).