I don't know what the "due for servicing" plan is .... But if a train arrived in Phila Greenwhich yard, and it was due for servicing, it might be shut down overnight, it would be taken to Pavonia NJ loco shop ASAP, and put on an incoming servicing track, and possibly shut down again, as it might take days or weeks to clear a loco inspection pit track for it to come into the shop
When it is serviced it would go to a load test device, and tested, and put on a outbound ready track, and shut down again, awaiting a crew. Allot of times a loco is taken off a train, and left idling for a recrew, and is immediately put back into service, or sent on a power move to a distant terminal where it is needed.
But in Trainz we don't really have to worry much about burning fuel and EPA requirements ... I like starting up scripted locos, but the majority with locos with these features is very sparse
The problem with disabling a loco with a dummy enginespec is, that you have to go back into the config file and re-enable the disabled loco ... or you are left with a permanently shut down loco