An effective standard system of Selective Door Operation (SDO) must be one of the keys to increasing capacity on the railways. The lengthening of station platforms to accommodate longer trains can be prohibitively expensive at some locations and is unnecessary at others.
The solution is to fit every door on every train with a detector linked in to the door-actuating circuit, to prevent it from being opened unless there is a station platform alongside. It might take the form of a light and photocell on the train, with the face of the platform being fitted with a strip of reflective material like that on car number plates. Or it could use some kind of sonic or infra-red device - the principle is the same. In the long run this would be a good investment as all trains could stop at all stations. It would be fail-safe, automatic and do away with the complexities of GPS location finding, which requires the train to "know" where it is.