Dear Riptof,
London underground use them for track inspections and works supervision (that which they do undertake).
Rail cycling does exist as a sport on abandoned railways. Charles Henry attained a record of 70km/h in his rail bike "Sapper" at Interlaken in 1999.
It is suggested that the world HPV record could be broken on a rail bike running on a special 200mm guage track. Bicycling Science by David Wilson has some more detail.
That said far better to get cycling and rail properly intergrated. As a useful step to which, I recommend signing the petition that Chadlington mentioned in the previous topic, assuming it is still live.
Yours, James
