When tunnelling, what happens when the cards drawn do not match those played?
If none of the drawn cards match the colour of the cards you chose to play, then you are in luck - the tunnel does not cost any additional cards. To put it another way, the tunnel costs 0 additional cards. This case isn't explicitly discussed in the rules, but I think it's quite clear. 


  When attempting to claim a Tunnel route, a player first lays down the number of cards required by the length of the route. Then the three top cards from the Train draw pile are turned face-up. For each card revealed whose colour matches the colour of the cards played to claim the Tunnel, an additional card of the same colour (or a Locomotive) must now be played from his hand. Only then can the player successfully claim the Tunnel route.