Have just been watching a a film clip showing the trials of the Properllertriebwagen Kruckenberg Schienenzeppelin (the Railzeppelin) in 1930; the aero industry engineering is evident in the early, unpanelled views (and is remarkably beautiful - 01:00-01:20 - could live with that as interior design). (Link: ) What I hadn't realized before is that, as originally built, the RailZeppelin was a straightforward, two-axle four-wheeler with a fixed wheelbase of some 19.6 metres (64' 5 3/8") - or more than double that of a Class 141 Pacer. This has set me wondering - what is the maximum theoretical wheelbase for a diesel / electric locomotive or self-propelled vehicle running on standard-gauge mainline, before it falls foul of the track geometry and ends up on the floor? Am assuming that the maximum permitted length would be a safe margin under that figure - but what else has pushed the boundaries and played havoc with the p/w? Apologies in advance if this is a well-trodden path; any recommendations for reading gratefully received.