Box tunnel subway is located approximately 150m in from the east portal of Box Tunnel in Corsham, Near Chippenham. The subway was created as a result of mining activities around 140 years ago, when an adit was driven beneath the tunnel. The extant adit in general consists of engineering brick abutments which support a Barlow Rail soffit and large riveted beams which support the running rails. As part of the electrification of the Great Western Railway, the track within Box Tunnel needed to be lowered by approximately 300mm. To enable this to be carried out over the subway, the existing Barlow Rail soffit and bearing timbers needed to be removed.