Interestingly, the tunnel is totally unnecessary from an engineering viewpoint and only exists because the Scotts of Sundridge Park would not allow access to their land during construction. In places the tunnel roof is just 4 feet below ground.

The railway to Chislehurst opened in 1865, joining the railway from Tonbridge to become the South Eastern Railway's main line in 1868.

Initially double track, demand for new stations and extra services as the suburbs expanded stretched the line to capacity.

Following the end of the company's feud with their rivals, The London Chatham and Dover Railway, and the establishment of joint management, money was found to quadruple the track including the building of a second tunnel through Elmstead Woods, illustrated here, and to open a new station to the south of the tunnel in 1902.