Board Game Arena

I would like to know how completing cards with a station on BGA works. Should a route be marked as completed immediately or will this only be visible at the end of the game?

For example: I have the route Cadiz - Stockholm in 3 player game but one of my opponents had the line Madrid - Pamplona before me. So I complete Cadiz - Madrid and Pamplona - Stockholm and place a station at Madrid. Should the route Cadiz - Stockholm be marked as completed immediately or only at the end of the game?

Only at the end of the game if it use station.
Full explanation here : https://en.doc.boardgamearena.com/Bugsc ... _validated

I take it then that the station can only be used to complete ONE ticket. If multiple of of my tickets pass through a route I only have because of a station,. only one will be completed?

That's not what the rules says. The station can replace only one route, but this route can be used by multiple tickets.