I know it's slightly off topic, but let me give you a real example of where a disabled person travels off route.
A blind friend of mine travels between Hereford and London occasionally. The quickest way to do this is often via Crewe, but this is not a permitted route. Permitted ways to go are either through the Cotswolds (of which there are only few direct trains) or via Newport, South Wales.
However my friend has a guide dog and there is no easily accessible place to exercise the dog at Newport (or at least, not one that he knows the way to). At Crewe there is a patch of grass just to the right of the station entrance, and my friend knows how to get to it, because it's a regular change point from Hereford to all sorts of places.
He can book assistance via Crewe for this reason. Train managers seem to be aware and no-one has ever queried it.
For people that are blind, or otherwise have a disability that precludes them from driving, the train is the only realistic alternative. As they have no choice, I think a discount is not only perfectly reasonable but that it actually enables people who are disabled to live their life. To that end, it should extend to all tickets right across the board.
