Construction difficulties brought delay to the extension of the Great Northern’s route from Thornton through to Keighley. The 1½ miles to Denholme was opened as a single-track goods line on 1st September 1882, incorporating two tunnels – Hamer’s Hill and Well Heads, the latter being significantly longer at 662 yards. The through route to Keighley didn’t appear until 1884.

The northern end of Well Heads Tunnel marks the highest point of the line – 887 feet above sea level. The structure boasts characteristic GN stone portals with buttresses either side of the entrance. Inside, vertical masonry-built walls – accommodating tablets and supports for signalling cables on the Down side – give way to a brick arch. Regular refuges, also in brick, are provided. There are no ventilations shafts.

Closure of the Thornton-Cullingworth section came on 11th November 1963.

Part of the northern approach cutting has been infilled since the railway was removed, effectively sealing the tunnel at that end. Its ownership is divided, with the southern section being in the hands of British Railways Board (Residuary). Work to repair spalled brickwork has taken place in the past 20 years.

It is hoped that the Great Northern Trail will eventually be extended northwards from Thornton to pass through the tunnel.

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