Can't help feeling that there is a general tone amongst some of the posters here that passengers are generally idiots who expect trains to be held up for their personal convenience, and who barely deserve to travel on our publicly funded railways at all.
Having some clarity about when you actually need to get to your train, instead of not knowing until you get to the station (and even then not unless you happen to see the relevant notice) is something that would improve the passenger experience without increasing delays or making anything more difficult for rail staff.
The only arguments I can see against it are
1) people will complain about trains leaving 'late' more than they do currently about the doors being closed 'early'. I'm not convinced this will be the case
2) adding 1 or 2 minutes to an advertised journey time will lose rail its competitive edge over other modes of transport. Really? Come on.
Dispatch times vary according to the type of train, the type of journey, the station, and possibly the time of day. How do we resolve this in the simplest way?
1) take the longest possible dispatch time and tell passengers they need to turn up this much earlier for every single train on the network. So people have to wait two minutes longer than really necessary for many trains. Either they repeatedly waste time or they realise this is the case and start ignoring the seemingly over-cautious advice, result being some missed trains and unhappy passengers.
2) stick a load of signs up at stations, each with different times stated, and showing information that passengers only recieve once they are at the station. Result being confusion and unhappy passengers
3) For each train departure, advertise the doors closed time that is actually necessary for that particular service and location. Result being that it's completely clear when passengers need to be at their train, happier passengers, and for rail staff the ability to point at a clearly stated time if any passenger should try and complain about having missed their train as a result of them cutting it too fine.
Seems pretty obvious to me what the best solution is...