What happens if you get off the train near an army or naval base?

You must have been at Lympstone Commando, on the line between Exeter and Exmouth. It is the only station of this kind in the UK.

British Steel Redcar, in Cleveland, and Ampress Halt, in near Lymington in Hampshire, were similar, in that they were only for the use of staff and visitors at those large factories and had no access to any public road.

Lympstone Commando station has no access except to the Royal Marines base there and so anyone getting off a train there and without permission to enter the base has to wait on the platform until another train comes. The base security staff might question anyone who did this and note their identities, in case they had done it on purpose hoping to observe what went on in the base (e.g. to plan a bomb attack) rather than having happened to get off there by accident.

Possibly because the station was at a Military installation and the stop was put there for military to have a better means of transportation available. Getting off would only cause you to stay at the station till the next train came in. You would not be allowed to go anywhere.

I would imagine there's nothing else around there. So if you get off the train and don't have a military ID that would allow you on base, then you'd pretty much be standing around in the middle of nowhere. Unless the stop was actually ON the base, rather than just near it as you say. If it was ON the base you'd have to show military ID to stay on it.

they would probably kicked u out of the military base area