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The rules for the base game are clear that you need at least one colored (non-wild) card when building a link to a new city.
Does this rule carry over to tunnels? For example, could I play a single wild on each side of the tunnel? Or do I need a non-wild on at least one side of the tunnel?
You may play with a single wild on each side of the tunnel. This is different than the base rule for connecting to a "new" city.
Similarly, does this rule carry over to switches? The rules imply that at least one side of the switch can have only wilds: "You must play at least 1 track card, colored or wild, on either side of the switch card." But it's unclear to me if you can have only wilds on both sides.
You may have only wilds on both sides.
Finally, an unrelated question about tunnels: Am I right in assuming that the track cards you play when building a tunnel score immediately, just as for any other link?
As always, feel free to house rule things if you feel something different enhances your personal experience :)