Combe Down Tunnel

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Combe Down Tunnel on the Somerset & Dorset Joint Railway is a narrow mile-long single-line bore with a grade of 1 in 100, on a closed line with a ruling grade of 1 in 50, below high ground and the southern suburbs of Bath, England at Combe Down.

1 Overview
2 Accident
3 Better design
4 Two Tunnels Shared Path
5 See also

[edit] Overview

The tunnel was on the last section of the Bath extension line built in 1874 which effectively bankrupted the independent Somerset and Dorset Railway 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 smaller Devonshire tunnel, led to the northernmost section remaining single-track throughout its working life. Trains heading south from Bath were often banked by a locomotive that detached itself from the train at the entrance to Combe Down tunnel, and then reversed back down the slope to Bath.

[edit] Accident

Combe Down tunnel had no intermediate ventilation and there were significant problems with fumes. On one occasion in November 1929, the driver and fireman of a northbound goods train were overcome by smoke, and the train ran away, crashing into the goods yard on the approach to Bath Queen Square railway station, killing the driver and two railway employees in the yard.

[edit] Better design

The fume problem in the Combe Down Tunnel was a consequence of the restricted bore, lack of ventilation shafts, and the gradient, - the last could have been eased at the design stage. However, determining how much easing would have been sufficient is hard to say, the best way being to examine tunnels locally and overseas for examples to check. Several other tunnels worldwide have caused engine crews to suffer in a similar way.

In the case of the Combe Down Tunnel, the top end of the tunnel is also a summit of the line, and any lengthening of the tunnel at that end tends to ease the gradient. However the S&DR was in bankruptcy, and this was not necessarily an option.