Originally uploaded by seadipper.
The train on the left is a class 319 Thameslink train with a BSI coupling. The train on the right is a Virgin Voyager with a Scharfenberg coupling. They can't be joined so if, say, the Voyager breaks down, the 319 cannot push it out of the way. And they cannot be attached to a locomotive - a special match vehicle has to be put in between.
Any ten year old with a train set knows that all the trains should be fitted with the same type of coupling. Seemingly the experts who are responsible for co-ordinating Britain's railways do not.