S-curve
Work in progress: This section is still a work in progress — content is being added and revised continuously.
What is checked? Where two curves of opposite direction follow each other directly, the Plan check verifies that a sufficient straight lies between them.
Why does it matter? In an S-curve the ends of two coupled cars swing out in opposite directions — the lateral offset at the coupler adds up, and close-coupling kinematics make it worse. The result: derailment or buffer locking, often only when pushing. Strictly speaking the governing quantity is bogie-pivot spacing plus twice the end overhang; car length is its safe approximation.
How bad is it? — the radius decides. The lateral offset at the coupler grows roughly with car length² ÷ radius: the tighter the radius, the more dangerous the S-curve. A broad reverse curve, by contrast, is harmless — main lines run them deliberately without an intermediate straight. The Plan check therefore grades tight S-curves more strictly (error) than broad ones (mild warning, suppressed entirely on secondary tracks).
Where does it occur? Most often at turnouts — precisely where the route is already curving (passing siding, crossover). If a turnout diverges against the curve, route curve, turnout curve and correction curve line up into a double kink (“double S”). Because the inflection point sits right at the turnout, it is easy to miss.
How to fix it: On open track: a straight of at least one car length between the curves — or widen the curves (a larger radius defuses the S-curve). At turnouts there are more elegant routes that create no S at all (picture above):
- Place the turnout inside the curve — the diverging leg continues the curve in the same direction (prototypical).
- Use a curved turnout — both legs lie within the same curve (geometrically smoothest).
- Mirror the turnout (swap handedness) — turns the double S into at least a single S.
Numbers & defaults: intermediate straight ≥ longest car (H0 default: 264 mm). A tightest radius of 600 mm or more (H0, scaled & editable) counts as broad (mild warning only). The notice weighs more on through main tracks than on slowly worked secondary/storage tracks — there it is an ignorable warning, not an error.