Proper use of stations, only as connecters/extenders or in place of built track?
The rules on stations are a bit complex. But there is nothing in the rules that either:


prohibit the use of two or more stations in a single route.
force the route to have at least one section without a station.


But since each unused station gives 4 points to the end score. And short routes don't give that much points. It is probably only wise to do, if the sections with stations are used with other routes.


  Building a train station
  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.  


So you can use stations to borrow a route from another player. Each station borrows only a single route.


  Stations may be built on any unoccupied city,
  even if it currently has no claimed routes into it.


This is a bit vague, but later on, it implies, you can build a station on an unoccupied city and in the end chose one of the routes (taken by another player) to claim.


  Two players may never build a Station in the same city. Each player may build a maximum of one Station per turn, and three Stations through- out the
  course of the game. 


(Skipped part on using cards to pay for stations)


  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. 


So it can be an advantage to claim a city using a station to avoid a later clogged bottleneck. Which also implies only one station can be build on a single route. (It does not solve the situation in which two players each claim a different city on the same route...).


  A player is never required to build any Stations. For each
  Station a player has not used, four points are added to his score at 
  the end of the game.