What size should a tunnel portal be in OO gauge?

A single-track OO gauge tunnel bore should be approximately 72mm wide and 76mm high. The portal (including wing walls and parapet) will be about 98mm wide and 96mm high. Commercial portals from Ratio and Wills are made to these dimensions.

How long should a model tunnel be?

A tunnel should be long enough that a train disappears inside completely. For OO gauge, a minimum of 500mm works for short trains. Longer tunnels add drama but need access for derailments — include a lift-off scenic section or removable hillside.

How do I ensure reliable running through tunnels?

Lay track on a smooth, flat surface inside the tunnel. Ensure no joints right at the portal entrance. Test extensively before adding scenery. Include inspection hatches or removable sections to reach derailed stock. Keep curves gentle inside tunnels.

What materials are best for tunnel construction?

Tunnel linings can be made from corrugated card or embossed styrene sheet. Use expanded polystyrene (carved) or plaster-soaked bandage over a former for the hillside. Commercial portals are available in stone and brick styles from Peco, Ratio, and Metcalfe.

ModelCalcs

Tunnel Calculator — Bore & Portal Dimensions

Calculate tunnel bore dimensions, portal size and minimum length at scale.

Gauge

Number of Tracks

Tunnel Length (mm)

Interior length through the hill

How We Calculate This

Calculates tunnel bore and portal dimensions from UK standard tunnel profiles, scaled to your gauge.

Formula

Bore width = Real bore (5500mm single / 8200mm double) / Scale ratio
Bore height = 5800mm / Scale ratio
Portal width = Bore width + (2000mm / Ratio) for wing walls
Portal height = Bore height + (1500mm / Ratio) for parapet

Frequently Asked Questions

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Last updated: February 2026

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