That's the Class 57 that has recently been converted to coal burning. Apparently it's to protect against further oil supply disruptions.
Why? Train Windows do open for Fresh Air on older non air conditioned stock Just the 57s don't normally clag like that
I suppose it depends on the people inside the coaches. Enthusiasts like a bit of clag and don't mind the smell of the diesels No different to steam locos - apart from a different smell
One of the things I miss from the loco hauled days of the 70’s and 80’s. The aroma of turbocharged diesel fumes wafting back into the train. Even better in the north of Scotland when mixed with the aroma of steam heat in a nice original Mark One coach. Probably not healthy but there’s far worse vapours you can put in your lungs!