A recent quandary came up with some friends in a game of Ticket to Ride. If, in the last turn, you have the cards to play a route longer than the number of trains that you have left, can you do so? Or are you limited by your number of trains?
No, you are limited to the 45 trains you started with.
From the rules, under Claiming a Route (emphasis mine):
The player may claim a route on the board by playing a set of Train Car cards that match the color and length of the route and then placing one of his colored trains on each space of this route
If you do not have the trains available to place on each space needed then you may not claim the route.
This is also how Days of Wonder's electronic implementation works.