Shouldn't be all that hard to get mark 4s cleared for other routes though, given that they were built to the same C3 loading gauge as the mark 3s. Not that there are all that many 25kV electrified routes which mark 4s haven't already been at least planned for (Grand Central and Grand Union plans for Blackpool, Stirling and Cardiff for example, plus TfW's actual operations) and from previous discussions on this topic it appears no diesel locos are able to work push-pull with mark 3 and mark 4 DVTs without modification (except perhaps the buffer-fitted class 43 power cars) so railtour mark 4s would be unlikely to venture beyond the wires much anyway.The problem is that the Mark 4 has a very limited route clearance, unlike the Mark 3s, where large portions of the network are cleared (more than you might think).