No, you are being narrow minded. If I walk up to your house from the street, do I get to the front first or the back first? Traditionally, when referring to things, the front is the first thing we come to. This is standard English usage
Except that, whichever way you approach, the front of the house is the front and the back of the house is the back. The same as a car or a bus. In the later case, irrespective of whether the entrance is at the back or the front!
With respect to two trains standing standing waiting to depart, if the platform entrance is at the buffers the front train is the second one you come to. If it is at the other end of the platform then the front train is the first one you come to. If it is the middle, then, unless it is a terminal platform when the first train is the one with clear track in front of it - which you may not be able to see, otherwise it can be anyone's guess!
Irrespective of the actual location of the "front train", it is quite possible that passengers who are not familiar with the system will make a mistake. Even "experienced" passengers in a hurry have been known to misread the departure boards and get on the wrong train!
To counter this, on the commuter trains out of Euston they used to lock-off the second unit and put a marker board on the platform at the end of the first unit. Mention of locking-off brings me back to the topic.
Again at Euston, on many occasions I and many others who have arrived early have stood for a considerable period of time by the closed doors of our train, waiting for the crew to arrive and open the doors, whilst on other occasions the doors have were open and so we could sit down. I never understood why.
Finally, about five years ago I spent 10 days on a grand train tour of England, Scotland and Wales. AFAIR, apart from the first trains from the terminus in the mornings very few were locked-off when we arrived, but the majority of those DMU's which were had the engines running ...