What clearance height do I need for a bridge over railway track?

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.

How wide should a bridge opening be?

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.

What materials can I use for model bridges?

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.

How do I model bridge abutments and approach gradients?

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.

ModelCalcs

Bridge Calculator — Span, Clearance & Approaches

Size bridges for your layout with correct span, clearance height and approach grades.

GaugeSets the scale ratio that divides every real-world dimension

Bridge TypeOver: deck carries a road above the track. Under: the line is raised on embankments to span a road or valley

Number of Tracks

Tracks passing through the opening (1-4); widens the clearance per track

Span Length (mm)

Clear span of the deck on the model, in mm

How We Calculate This

Scales UK structure-clearance figures to your chosen gauge, then sizes the deck and the embankment approaches needed to reach it.

Formula

Clearance height = Clearance height (default 4720mm structure clearance) ÷ Scale ratio
Clearance width = (Clearance width per track ÷ Ratio) × Track count (the 3200mm per-track figure already includes side clearance, so no extra gap is added)
Approach length = Embankment height × Gradient denominator (run = rise × N for a 1-in-N grade)
Total length = Span + (2 × Approach length)

Scale ratios used: OO 1:76.2, N 1:148, O 1:43.5.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify quantities before purchasing materials.