Each station allows you to claim a single route. 

That route may be used to fulfill more than one destination ticket, but every ticket must use the same route.

From the rules,

> A Train Station allows its owner to use **one, and only one, of the routes** belonging to another player, into (or out of) that city to help him connect the cities on his Destination Tickets.

> If a player uses the same Station to help connect cities on several different Tickets, **he must use the same route into the city with the Station for all of those Tickets.** The Train Station owner does not need to decide which route he will use until the end of the game.

(Empahsis mine.)

For example, say you had the following three destination tickets:

* Frankfurt – Smolensk (Complete save for Frankfurt – Berlin) 
* Amsterdam – Wilno (Complete save for Frankfurt – Berlin)
* Essen – Kyiv (Complete save for Essen – Berlin)

With a station at Berlin, you could complete either the first two destination tickets or the third, but not both (unless you also had a station on Essen).

[![Clip of the relevant portion of the board][1]][1]

  [1]: https://i.sstatic.net/bJNGn.jpg