I'm going to go against the grain here and say as a "barely railway" person, I've actually quite enjoyed it so far, taking it for what it is - a drama series with filming constraints and a slightly fantastical plot line (but again, that's the whole idea) - and so there are going to be things that "railway people" will pick up on. How many spare 387s are sitting around for a BBC film crew to set up and use so they have the right train set available? How many spare Caledonian Sleeper carriages are there? "We are sorry to announce your train has been cancelled owing to the BBC using it for filming"...
Lots of comments seen on the 321, but none whatsoever on the fact there isn't a train operating company called "Heart of Britain", which seems a curiously pedantic approach. I wonder how many complaints the BBC would get for unrealistic filming and overuse of CGI if the train didn't go "clickety-clack"! (It's a bit like wondering why no TV murder mystery ever seems to be able to cope with just one body since about 2005...)
Not entirely sure the Guardian reviewer watched more than the first episode, because her "banging on the door" comment is fairly obviously visible and explained in Ep.2.
I agree there are certain characters you aren't meant to warm to - nasty politician lady being high up on the list (will we find out what her scandal was?), and I suspect there is something behind the rather stereotypical portrayal of the oil rig worker, too (just ever so slightly overdoing the "I'm an oil rig worker" look?). I last saw Joe Cole playing Jan Gies in "A Small Light", a very different sort of role set in a very different era (1940s Holland), and James Cosmo usually plays quite strong characters in things I've seen him in, so interesting to see him look slightly 'weaker'.
Ultimately, it's - of course - all down to personal preference. I pretty much hated McMafia and The Honourable Woman, both of which fell into that same 9pm Sunday evening drama slot, and both of which received pretty rave reviews from elsewhere. As for Nightsleeper, I'm tossing up whether to wait until Sunday for part 3 or head to iPlayer...