Bridge Calculator — Span, Clearance & Approaches

The default 4720mm (15ft 6in) above rail level is the UK structure/electrification clearance — the height a new overbridge needs to clear 25kV overhead wires, not the train envelope itself. Divided by the scale ratio that is about 62mm in OO (4720 / 76.2) and about 32mm in N (4720 / 148). If your line is not electrified you only need to clear the train: the UK loading gauge is roughly 3962mm (13ft) above railhead, which is about 52mm in OO — many modellers use a practical 54-60mm minimum to allow for slightly over-scale stock and derailment access. Always add a few mm for track underlay and safety.

For a single track in OO the minimum opening is about 42mm (3200mm / 76.2). The 3200mm figure already includes side clearance, so double track is simply twice that — about 84mm — which is why this calculator multiplies the per-track width by the number of tracks. Then add wall/parapet thickness (5-10mm each side) for the bridge structure itself.

Card and balsa wood for scratch-building. Styrene sheet for clean modern structures. Plaster for stone bridges. Commercial kits from Ratio, Wills, Metcalfe (card), and Peco cover most common UK bridge types. Laser-cut wood kits are also excellent.

Abutments are the supports at each end of the bridge — make them wide enough to carry the deck and tall enough for the required clearance. The approach length here is the embankment height multiplied by the gradient denominator (run = rise × N for a 1-in-N grade). Prototype rail gradients are gentle (1-in-50 to 1-in-100) and would need very long ramps, so the default is a compressed 1-in-15 typical of model layouts. Lower the gradient number for a steeper, shorter approach; raise it for a gentler, longer one.