To be fair, I can completely understand the issue people have with the racket and pollution from running diesel engines when the vehicle is not moving, and there really needs to be legislation to prevent this and require the installation of shore supplies, as are more the norm in other countries. I do get that knackered old heritage DMUs needed leaving running as otherwise they wouldn't start, but that just isn't true of a modern DMU.
Electrification is of course the real solution, but in the meantime leaving engines idling for more than a couple of minutes, particularly at night, when no passengers are on board is simply an unacceptable practice and needs to be outlawed.