This is a bit of a small detail that most players won't notice or mind about but this is a large inaccuracy. In real life the computer controlled signals or the signalman will change the signal after the train has cleared the signal light no matter the length, but in TSW2 the signal changes right after the front of the train has passed. This is a small detail that makes the gameplay unrealistic. On TVL the signal changes to red after the train has cleared the signal post which is accurate. I've seen a lot of British railway videos and the signal changes after the train has cleared. In Germany most signals change once the train is clear of the signal. Not sure about the USA though. So why not in TSW2?
While I agree that the signals change to red too quickly (literally the second the train passes it), they don't necessarily do it only once the whole train has passed either. In Germany (in a simplified way) there are two types of signals, the ones that change once the front of the train passed the signal (after a couple of seconds delay of course, these are mostly block signals outside stations) and signals that only change once the end of the train passed (mostly signals in stations).
I don't really have much time to observe signals outside of stations, sorry if I got some informations wrong
Years of hanging around train stations both for pleasure and going to and from work in the UK can definitely say with total conviction that the signals change back to red before the end of the train passes then, sometimes after literally the loco or first carriage of the multiple unit, in that respect it's fairly realistic.