In 4 and 5 player games, both tracks of a double-route can be claimed, though not by the same player.
In 2 or 3 player games, once a player claims one track of a double-route, the other track can no longer be claimed by anyone.
In double-routes pairing a gray track and a colored track, a player may use a set of Train Cards of any one color (including the same color as the colored track) to claim the gray track.