I'm firmly convinced that diesels will have all but vanished from our railways by 2030. This is because of Peak Oil, which will make the running of these machines totally uneconomic. We'll see all railways either electrified or run by a new breed of steam locomotives, using super efficient wood burning as power.
I'd encourage all railway enthusiasts to photograph this transitional form of transport whilst they still can! Once they are gone they really will be gone for good.