For me, one of the biggest problems is getting from the platform onto the train.
We are enthusiasts, we (often!) know what we are doing, we are generally confident enough to stand at the right platform etc.
But a lot of members of the public aren't, they'll find this anxious - am I standing in the right place? And then, when the train turns up, am I standing at the right end? You might have a reserved seat in Coach B but no idea of whether that's going to be at the start or the end (or even if there's a Coach A, since some trains don't have every letter used). Even if you are standing at the right end of the right platform, will it be a train with doors at the ends or closer to the middle of the carriage? Will the furthest carriage be at the far end of the platform, or should I stay near the middle of the platform and hedge my bets? Do I have to wait until the train comes in to try and see whether the front carriage is "A" or "J"?
For me, that's a lot of things to go wrong, a lot of things to be anxious about. If I go to an Airport or to the Cinema then I'll have people pointing me in the right direction but we don't have the luxury of sufficient staff at stations to help everyone with such matters.
Part of the problem is that it can change a lot from station to station or even between different services using the same station (e.g. sometimes the screens will tell me that First Class is at the rear, several minutes before the train arrives, but sometimes they won't).
If you can find something to improve this then you'll make life a lot easier for non-regular passengers (bearing in mind that, once you take every day commuters out of the equation, most members of the public take no more than a couple of return journeys a year - certainly outside London), but you'll also improve dwell times, which will make services more reliable.
The example I've given before is Stagecoach Supertram, where the trams stop at each station in line with the markings on the platform, so people know where to queue up, meaning you can let a dozen people off and have a dozen board and be off again pretty swiftly. The Jubilee Line extension in London means a tube train can soak up hundreds of passengers and zip away again in next to no time.
Whilst I appreciate that a one/two hundred metre long heavy rail service is fundamentally different (given reserved seats, different classes of accommodation etc), the scrum at some major stations is pretty off-putting - as anxious passengers push past each other, looking for different bits of the train (witness how many people will start off standing near the middle of the platform and only realise once the train is coming in where the front carriages start).
Different people have tried to "solve" this - e.g. Virgin had different coloured "zones" at stations - but since the TOC operating the service may not be the people operating the station, that doesn't always work. I'd be interested in anyone's idea on how we could make boarding much simpler (and therefore make station dwells more reliable and potentially even faster).