wouldn't it be easier to just change the timetables and close the doors at the correct time!
That would make sense if all transport operators did the same thing. My flight e-tickets say the plane leaves at 1430, but I can't turn up athe airport at 1425 and expect to get on, even if the flight is late leaving.
The Swansea to Cork ferry timetable shows it leaving at 2100, but I can't pitch up at the terminal at 2050 and get on.
In both these cases, it is because the doors will have been closed and the pre departure check will be taking place ready for a prompt departure.
Of course, there are no check ins necessary on the railway, and this is a great benefit, but I honestly don't see how changing timetables would help. It wouldn't stop trains leaving early. Neither would it prevent people being late for their train for whatever reason!