I remember some old materials about the class 180s (which I believe use the same or similar engines to the 221s and also have one under each car). These materials were aimed at traincrews and I believe they gave the advice that if the engine on a particular car wasn't running after starting the engines from the cab, they could try to start it using a manual procedure (involving pushing buttons in cupboards with specific numbers; all went over my head a bit really as obviously I'm not a trained driver!), but only "if time was available".
The implication being, of course, that if the service was running late it was considered better to just run with an engine out than spend time attempting to start it. This seems counter-intuitive as you'd presumably want as much power as possible to make up as much time as possible, but presumably they thought it was powerful enough even on only 4 of 5 engines.