A (possibly) Turing complete language:
As the validity and the semantics of a program depend on the structure of the London underground system, which is administered by London Underground Ltd, a subsidiary of Transport for London, who are likely unaware of the existence of this programming language, its future compatibility is uncertain. Programs may become invalid or subtly wrong as the transport company expands or retires some of the network, reroutes lines or renames stations. Features may be removed with no prior consultation with the programming community. For all we know, Mornington Crescent itself may at some point be closed, at which point this programming language will cease to exist.