Rules about claiming a tunnel in Ticket to Ride Switzerland
You must have an additional Train Car card 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.