I have mentioned it before, when I was in Stoke Control in the 60's & 70's, there was a chronic shortage of class 47 diesels - the "booked" loco's for Broad Gauge - LM inter regional freights. Just to keep trains running, you used what was available. If it was possible to replace a 47 with traction that came back "LXC" then you had another chance of keeping the job running. We (OK, I was) were just as guilty, dropping a 40 onto trains into Broad Gauge territory. Sometimes the ploy did not work, and a driver was found that knew the traction. Do not forget that 52's were built at Crewe, and quite a few men knew them. One odd fact being if the driver was working his rest day, he tended to sign more routes and traction than on a normal (just what were they?) day. Drivers also moved about the country, as I recall when we lost a 40 for a couple of days because an ex Bescot driver went native and moved to Severn Tunnel Junction.
Only now can I see the good times that those days were. At the time it was very stressful - to put it mildly.
Mike