It's been happening ever since movies were invented, hasn't it? Film makers have always thought that a train is just a train and they're all the same.
When they use stock footage the results can be hilarious, with a Great Western train turning into an SNCF express and then into the Flying Scotsman in the space of about ten seconds screen time.
Alfred Hitchcock is guilty of this carelessness, as are many others and - yes - it drives train enthusiasts nuts.
I can't say I've noticed the errors in these programmes, but I probably haven't been watching closely enough. I'm sure you're right.