The fabulous news about the Adalantes is that they have selective door opening so they COULD stop and Melksham and have only doors at the platform opening. With the 125s, I've been told that they can't stop at Melksham because there's no way that the doors can be opened on the section of the train that's at the platform only.   

This is a very odd business - you may have come across many other cases of trains such as 125s stopping at short platforms. The difference is that in the case of Melksham the station was closed from 1966 to 1985 and re-opened under different laws /  a different health and safety, so in "our" case it's regarded as unacceptably dangerous to release door locks when a coach isn't alongside the platform.

Yes, trains can only stop to load and unload passengers at Melksham station if either the train is shorter than the platform or the train doors are set up in such a way that the staff can allow ONLY those doors that are alongside the platrotm to be opened.

This means that 125s cannot make a passenger stop, and it's also prevented special trains calling to pick up or drop off passengers.

All allowed because those stations were opened many, many years ago under a health ans safety regime that was more lenient.  Melksham was (re) opened in 1985 and the conditions that apply are the regime of that time.