AIUI it is a load of the trains dropping a unit at Carlisle and that unit popping to the depot?
i had thought that the units were splitting to go to both Glasgow and Edinburgh though
Yes, the intention is that every half hour a 400m train runs from Euston to Carlisle, then splits with one portion to Edinburgh and the other to Glasgow. Obviously first/last trains etc may follow a different pattern.
Assuming your platforms are large enough, the length of train should make little difference to the time to disembark and embark passengers.
It'll also depend on how well passengers spread down the platform. If NPR is turn up and go (no seat reservations etc) the last thing you want is everybody boarding at Piccadilly all crowding into the two seats of doors nearest the buffer stops rather than spread along the train. Station design (like London Bridge and Reading) can help with this by designing where passengers feed onto platforms.
