In BR steam days, would a train always have been pulled by the same loco or type of loco? So for example, an express, Paddington to Penzance... Would it always have been a King? or would other locos have done it occasionally? (a castle or 2 manors for example?) this info is not available in the working timetable, and i am too young to have seen mainline steam. Thanks for your help.
It varied a bit between regions, but yes, Locos were diagrammed back then, just as they (or HSTs, DMUs etc) are today.
So yes, a train would be diagrammed for a locomotive in a particular power range. On the WR (not my speciality) Kings were usually kept back for the heaviest expresses on the Wolverhampton and Plymouth lines. Of course, sometimes, to suit operating needs, a King (8P in BR terms) might be put on a CAstle (7P) turn, or a Castle or County (6MT) might be put on a Hall (Class 5, I think) turn.
(I believe for some years in the50s, the Bristolian was limited to 7 coaches for a Castle, except on Fridays, when it was increased to 8 coaches, but given a King to manage the extra weight.)
Equally, at times of power shortages, or failures, you could easily get a Hall or perhaps a Grange on a Castle turn - but the train would probbly struggle to keep time if it was fully loaded.
But you would not normally get two Manors on for (say) a Castle, because the reason for Manors' existence was for some power on lightly laid lines - eg the Cambrian - which had strict restrictions on axle loads - Castles, Kings etc nor even Granges were not allowe on the Cambrian.
It was similar on other regions - although whoever actually allocated the locomotives knew the practical - as opposed to the theoretical - vagaries of the locomotives. So, for example, several sub classes of A2s - while in theory equal to their sister A1s and A4s - were not liked on express passenger work, and generally found themselves on less onerous turns.
I think enginemen actually preferred an A3 (theoretically only a 7P) or even V2 (6 MT I think they were) to some of the A2s.
As diesels came in, you had some unusually powerful diagramming. For example, on the Southern come 1962, you might find a Schools - a 3-cylnder express passenger 5P rating - on a Redhill-Reading Southern train of 3 or 4 carriages which could have been comfortably handled by an N or U 2-6-0. Likewise, I once saw a Jubilee - it was Atlas, I think - on a three-coach Northampton - Leicester local at Wellingborough. Such super power would not have normally been seen even a year earlier.