Hello. I thought I'd create a thread on the worst and least polluting UK diesel trains currently in passenger service and see what different views are said. There's no wrong or right answer . In my opinion, the worst polluters are the Class 142 and 143 Pacer trains (although the 143's top a 142). The least are MTU-powered HST's, Class 800 and 802 and Class 165 and 166 Turbo trains on a good day. That's the only problem I find with Pacer trains. I'm not sure why but those Perkins engines, even at 25-30 years old hardly chuck out anything most of the time when at idle. That's probably why the TfW Class 158's with Perkins engines would have been allocated to Birmingham services instead of Cummins-powered Class 158's ages ago. But the greenest diesel trains have to be the Class 800 and 802 trains - catalytic converters which I'd rather smell than from older diesel trains.