Combe Down Tunnel
Combe Down Tunnel is on the now-closed Somerset and Dorset Joint Railway main line, between Midford and Bath Queen Square, below high ground and the southern suburbs of Bath, England, emerging below the southern slopes of Combe Down village. Opened in 1874, this mile-long disused railway tunnel was once the UK’s longest (1,829 yards) without intermediate ventilation. It is planned that the tunnel will form part of the £1.8 million Two Tunnels Greenway walking and cycling path due to open in 2012. Its custodian is Wessex Water.

The tunnel was on the Bath extension line of the Somerset & Dorset Railway, built in 1874. The extension effectively bankrupted the independent company. 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. Trains heading south from Bath were often banked (assisted in rear) by a locomotive that detached itself from the train at the entrance to Combe Down tunnel, and then returned back down the gradient to Bath.

Combe Down tunnel had no intermediate ventilation and there were significant problems with fumes. On one occasion in 20 November 1929, the driver and fireman of a northbound goods train were overcome by smoke. The train was moving very slowly in the tunnel due to a heavy load and due to starting from a standstill at Midford. The 7F 2-8-0 locomotive plodded on and eventually breasted the summit of the gradient. Its downward course to Bath was accomplished more quickly, and the train ran away, crashing into the goods yard on the approach to Bath Green Park railway station, killing the driver and two railway employees in the yard. The fumes that overcame the footplate crew were a consequence of the restricted bore, lack of ventilation shafts, the exceptional humidity and lack of breeze, and the very slow speed of the train, running tender first. The inspecting officer, Col A C Trench recommended that maximum loads should be reduced or assistant engines provided to prevent a recurrence .

Two Tunnels Shared Path
This section of the Somerset and Dorset Railway, including the tunnels, is proposed for a shared-use walking and cycling path, and planning permission for this was approved in May 2008, with much of the funding via a Sustrans 'Connect2' lottery grant. The fourth and final £100,000 tranche of Council funding is due in financial year 2011/12 when the tunnel will be equipped with a cycle-friendly surface, mobile phone coverage and lighting related to motion detectors. Bat-friendly LED technology is being considered as an alternative to high-pressure sodium lamps.