Combe Down Tunnel is on the now-closed Somerset and Dorset Joint Railway main line, between Midford and Bath Green Park railway station, below high ground and the southern suburbs of Bath, England, emerging below the southern slopes of Combe Down village.

The extension line was later made double-track northwards from Evercreech Junction to the viaduct at Midford, but the substantial civil engineering works associated with the tunnel and the steep approach into Bath, including the shorter Devonshire Tunnel, caused the northernmost section to remain single-track throughout its working life.

The inspecting officer, Colonel A. C. Trench recommended that maximum loads should be reduced or assistant engines provided to prevent a recurrence.

[5] This section of the Somerset and Dorset Railway, including the tunnels, is now incorporated into a shared-use walking and cycling path.

The fourth and final £100,000 tranche of council funding was made in the 2011/12 financial year; the tunnel was equipped with a cycle-friendly surface and LED lighting.