The Queensbury-Keighley line entered Denholme from the south via two tunnels - Well Heads and Hamer's Hill - the latter being on the immediate approach to the station and its yard.
Both portals have a characteristic Great Northern design. Inside, the bore curves gently to the east on a downwards gradient. The lining involves vertical masonry sidewalls inset with brick refuges; the arch is also in brick. A drain is set at the foot of the west wall.
The 153-yard tunnel became operational as part of the Thornton-Denholme section on 1st September 1882. Closure came on 11th November 1963, although the passenger service had ended in 1955.
Today, the entrances are bricked up but both have doorways. The approach cuttings have been partially infilled, creating quagmires. The lower (northern) end of the tunnel is consequently choked with mud. The atmosphere inside is extremely 'thick'. Although some spalling of brickwork has occurred, the structure generally appears in decent condition.