Depends-on most stops the train arrives from further on the line-stops for maybe 2-3 mins, varies with size of station and timetable and then doors close and -whoops-off again. Even on stops where the traintrip actually starts doors will generally only open maybe 5-10 mins before, but this varies per country.
About anywhere where REServ is optional (like anywhere in DE=Germany, except overnight trains), ALL seats have-on top, on the side or somewhere, small screens that will tell from where to where that seat is reserved. That is-if it works, as with about anything anywhere on any train/railway, this is never ever 100% certain. What you can be reasonably certain about -esp. in DE, is that arrival will be late-maybe 1-2 mins, but 1-2 hrs can also happen.
Trains are not planes-there is NO checkIN, no boarding pass/procedure, noone will take you by the hand and guide, it all works like you maybe have ever experienced in the ´subway´ (we call it metro here) in NYC or LA. If it is really very busy it may mean that latecomers have to stand-and as about any train has many stops along the way, at every stop there will be new chances to find vacated seats.
Be reassured - about all of these newbees fears (esp from USAers) are in practice superfluous. You´ll get used to it in 2-3 trips.
The major prob (from experience, it sometimes still catches me in stations I´ve never been before after 40 yrs of using trains) may be finding the right track/platform and how to get there. Every country has its very own special system for that. Sometimes (mostly for very minor lines) they may be hidden behind the last fence or in a deep tunnel underneath, or in Swiss f.e. on the street)
But many, many more times I´ve rushed into a station from a tram/bus and caught a train that was bound to dep. in 2-3 mins, occasionally even trains that were supposed to have left already-but were late running!
On busy stations there might be trains leaving from same track rather close in time to each other so make sure that you get on the right train if you are coming early to the platform.
I once made this mistake at Zürich Hbf in Switzerland. I was at the right track about 5 minutes before departure and there was a train coming. I assumed that it was my train and I enterred. I ended up in the train depot and got help from a friendly train driver who took me with him in the locomotive to a station from where I could take a train to my destination.
