I am not convinced that filling with diesel as is complex as "the railway" seem to think it is.
Diesel fuel is non volatile and low risk. Car and truck drivers handle it routinely without special training.
Car drivers are even allowed to handle petrol without training, a far more dangerous fuel.
Having had the pleasure of visiting Rossmore Road Depot, adjacent to Marylebone. in the days of Class 115
DMUs▸ on the Banbury and Aylesbury routes, I can confirm that the DMUs were routinely refuelled by drivers - moved from siding to fuelling road, and either back again or out into traffic, with the driver getting out from the cab and doing the job. I'm not sure if this was official, or simply a convenient way of working. But it was definitely done.