Do you have to have the destination card in hand to claim the destination?
You can claim any unoccupied route between any two locations (that does not require passing through a third location) at any time. You do not require a destination ticket showing either location, and it doesn't have to be connected to any of your other routes. You only need:


Sufficient trains to cover the route.
Sufficient cards to pay for it.
You don't already control the other route between the same locations.
No other player controls the other route between the same locations in [2-3 players only].*




* Note that the number of players that restrict the use of additional routes varies between versions of Ticket to Ride. For example, extra routes are only blocked in 2-player games of Ticket to Ride: New York and Switzerland.