Are the gray double routes usable in 2/3 player games?
In standard Ticket to Ride? The double routes are only for 4 and 5 player games. Only a single track from the route can be used.
Important Note: In 2 or 3 player games, only one of the Double-Routes can be used. A player can claim either of the two routes between cities, but the other route is then closed to other players.
This varies in some the of the map collection expansions.
So yes the double routes are useable, but no you can't use both of the double routes only a single line, regardless of the route colours or lack of them.