I just want to be clear about how to claim a short tunnel route. Say I am taking a tunnel that requires only one card (red, for instance), if I turn over a red card I need another red (or a locomotive) to match it. But if I turn over 2 further red cards (ie 3 reds in all) do I need further reds or locomotives to match? It would seem strange to have to use 4 red cards in total to take a one card route.
You must have an additional train of the appropriate color (or a locomotive) for each card of that color that gets flipped over.
From the Ticket to Ride: Switzerland rules (emphasis added):
When attempting to claim a Tunnel route, a player first lays down the number of cards required. Then the 3 top cards from the Train Car card draw pile are turned face-up. For each card revealed whose color matches the color used to claim the Tunnel (including locomotives), an additional card of the same color (or a locomotive) must be played to successfully claim the Tunnel.