Between 1905 and 1908, Crystal Palace station was included in a proposal for an unusual new form of underground railway, the Kearney High-Speed Tube, devised by the Australian-born engineer Elfric Wells Chalmers Kearney. He envisaged the construction of a tunnel which would run from Crystal Palace to Cricklewood in north-west London, with a branch line terminating at Strand. It was to be operated with an unusual monorail system patented by Kearney which would be powered by gravity, like a type of underground roller coaster. Kearney failed to attract support for his scheme and the line was never built.
(I hope that this is acceptable, as it runs between two existing stations)