How was it decided which loco would pull which train: was the driver assigned one, or a class?
It depends on all kinds of factors:
1) What route is the train to take? Imagine it’s Glasgow Queen Street to Fort William in the late 70s/early 80s.
2) What class/classes of loco are cleared to run over that route? The WHL is a tightly curved route with relatively weak bridges and earthworks. As a result, it can only accommodate locos which are RA 5 (this was the case in BR days certainly) without requiring severe speed restrictions.
3) The only RA 5 locos on the network are 20s, 25s, 26/1s, 27s, 31/1s, 37s and Deltics. But they will need train heat and braking systems which are compatible with the stock which is to be used. The booked drivers will need to ‘sign’ it- i.e. know how to drive it, know its performance and handling characteristics (as every class of loco performs differently) and know how to do simple fault-finding if a fault develops in service. The loco also needs enough power to be able to shift the booked load at a reasonable speed.
4) 20s can be ruled out as no train heat and not powerful enough. 31s are a non-starter as Scottish region has no allocation*. Deltics only signed by Haymarket men among Scottish drivers, who do not sign the WHL. Eastfield has an allocation of 25s but the problem is that Fort William men do not sign them. 26/1s widely used on Scottish Region but Fort William men don’t sign them (and probably underpowered for the heaviest WHL trains, which also require some faster running on the North Clyde Line between Dalmuir and Craigendoran). So the choice is either a 27 or 37.
Basically, the loco needs to be cleared for the intended route (this is where the RA number is considered), the booked drivers need to know how to drive it, and it needs to be able to provide the appropriate form of train heat to the intended rolling stock (if a passenger train), and in all cases it must have brakes which are compatible with the rolling stock.
An individual driver would have either one class on his traction card or many, depending on the depot at which he was based and the link that he was in. For instance, a Mallaig driver in the late 70s would sign only 27s as this was the only class ever likely to be used on his diagrams. An Eastfield driver in one of the higher links might work semi-fasts and expresses to Dundee and Aberdeen as well as passenger trains up the WHL and Oban line, so in the late 70s might sign classes 20, 25, 26, 27, 37, 40 and 47.
If you’re interested in the north Highland routes as your name suggests, then Wick and Kyle drivers in the late 70s would only sign 26s, while a top-link Inverness driver would probably sign 26s, 27s, 40s and 47s (as he would work to Aberdeen and Perth as well as Wick and Kyle); an Aberdeen or Perth man would have a similar traction card to his Inverness counterpart.
*I love 31s, but I could probably imagine a Scottish region driver’s reaction to one if ever one were allocated there. Heavier than a 37 but probably no improvement on a 27 (and far less efficient).