I've played this game with the same people a number of times, and we've gotten to the point where we are more willing to rent other people's track to save money. It also makes the game more interesting when you start considering the benefits of renting track.
I think it would be a good optional rule that if someone else's track is only one move away from a town you need to get to, you should be able to rent that players track. But the person who owns the track must immediatetly (during your turn) build the one link of track that you need in order to reach the town.
The end result is that the cost of connecting the town is spent out of the 4 dollars you give the player to rent. This will also allow ports to see more use. This would increase the use of track renting and boating.
Making players build track is a bad idea. But, its not unusual for players to say "I'll ride your track for x turns if you build to ____."