Tunnels are surrounded by intricate black outlines, and when claiming them, you must take 3 cards off the top of the train card deck. Then, for every color card that matches the color of your route, (or the color you claimed a gray tunnel with) you must pay another card of that color. If a joker appears, you must pay for that as well. You can pay with jokers too. If you cannot pay the cards, you cannot claim the route this turn (you may try again though).
In Switzerland, jokers can only be used to claim tunnels.
In Team Asia, they have numbers on them. Instead of 3, take that many cards from the pile.