I imagine some faults can be fixed in minutes but need to be done in a depot.
I don't think that follows.
If something can be done within minutes it is certain that there's not much to be done beyond plugging in a laptop. Simply undoing
anything physical (with locknuts, or castellated nuts and split pins for example) and re-fixing will take a quarter of an hour. I bet even wrestling a burst brake hose off and re-fitting a new one will be a similar length of time.
All of these
could be done "in the field," but the time it would take to get someone there - with the correct part - and the possession needed on the adjacent line would probably count against it on today's railway. (Apart from the laptop job of course, and even then with a slimmed-down Longsight staff I wonder whether they could afford to jeopardise their shift deliverables to do a fire-fighting/rescue job that probably isn't in their contract?)