Tunnel Calculator — Bore & Portal Dimensions

At OO gauge (1:76.2) a single-track tunnel bore works out at roughly 72mm wide and 76mm high (from a real bore of about 5.5m × 5.8m). The portal, including wing walls and parapet, is about 98mm wide and 96mm high. These figures are for OO single-track at the default bore dimensions — switch to N or O gauge, or to double track, and the calculator rescales them. Commercial portals from Peco, Ratio and Wills are made close to these sizes.

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.

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.

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.