It's a thing which strikes me as comical -- and would appear maybe not altogether uncommon -- how in on-film "historical drama": great lengths are gone to, in quest of getting certain matters as near totally right as possible; but as regards other matters (frequently, it would seem, railway ones) -- very much not so. In the 2017 thread Errors you've noticed in films, which I cite in post #5: the TV series Heartbeat comes in for a good deal of "stick" over this issue. Posters lament the producers' being meticulous about "the right Corn Flake boxes, and accurate prices in the pub"; and their having "a flock of sheep specifically for the show, which were swapped with the usual ones for filming, as the breeds had changed since the 60s"; while perpetrating flagrant rail-related howlers such as "passenger trains on a North Yorkshire branch line are hauled by a Schools running tender-first"; and "an A4 in Express Passenger Blue, a regular performer through Aidensfield -- a bit overkill for the four Mk 1's it was hauling"; and an episode set at the time of the moon landing in July 1969, but featuring what clearly represented a regular passenger steam working -- er, the very end of BR steam took place nearly a year previously.Regarding historical accuracy in TV programmes, perfectly valid points made above, however it is rather irritating when the blurb for a new series says something like 'the producers consulted military experts to get the WW2 uniforms exactly right' but when a train appears it is hauled by a BR Standard!
Different people will have different responses to this kind of thing: some find it, basically, just too trivial to matter; some are genuinely aggrieved and incensed; some enjoy the comedic aspect of the anachronisms and errors... nonetheless, it does seem at least a bit odd, the way certain makers of these offerings are hyper-conscientious and diligent re the authenticity of some elements; and highly slapdash, concerning that of others.