Ted,
Go back to that railway web page and there is a link to a BBC Wiltshire website, where the underground government site was visited by them, there is a video of being driven around the complex and even a detailed map. From the several hundred comments posted on it, from locals and others that have worked down there, such as GPO, builders, RAF, etc..........not one has mentioned steam engines !
Also if that guy who supposedly worked on the steam engines, had driven one of the jeeps home when he retired..........let us see it.