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.
Freight 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 down the gradient to Bath.
 This operation was a very rare example of two trains being permitted to run within a single-line section at once, although the train engine carried an electric tablet and the banking engine a staff, both of which had to be returned to their appropriate signalling instruments before other trains could be dispatched into the section.
 Sometimes the banking engine would be conveying additional goods vehicles for Bath Co-op Siding (situated within the single line section), so the bank engine (carrying the bank staff) would shunt the siding on its way back to Bath Junction whilst the main train (with the single-line tablet) would continue on its way to Midford.
This unusual method of working operated right up to the closure of the S&D in 1966.
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. Freight 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 down the gradient to Bath.  This operation was a very rare example of two trains being permitted to run within a single-line section at once, although the train engine carried an electric tablet and the banking engine a staff, both of which had to be returned to their appropriate signalling instruments before other trains could be dispatched into the section.  Sometimes the banking engine would be conveying additional goods vehicles for Bath Co-op Siding (situated within the single line section), so the bank engine (carrying the bank staff) would shunt the siding on its way back to Bath Junction whilst the main train (with the single-line tablet) would continue on its way to Midford. This unusual method of working operated right up to the closure of the S&D in 1966.
Combe Down Tunnel