My wife and I play a lot of Ticket to Ride: Nordic Countries but recently we discovered we'd been playing Ferries incorrectly. Oops!
A ferry is depicted on the board as, e.g., 4 red spaces and 2 more red spaces with locomotive motifs. We'd been playing under the assumption that, to claim this route, you needed to play exactly 4 red cards and 2 locomotives. It seems, though, that there are several fine points to the rules that we hadn't noticed.
(a) You can always use any 3 cards in place of a locomotive. (b) You can always play an extra locomotive in place of a coloured card.
So, for instance, you could buy the abovementioned red route with 3 red cards and 3 locomotives, 1 red card and 5 locomotives, or even 6 locomotives. You could also buy it with 4 red cards and 6 blue cards.
Our big question arising from these revelations: do these rules stack? That is to say, can I buy this route with 3 red cards and 9 yellow cards? Eighteen assorted cards none of which are locomotives and reds?
Tunnels I think - I hope - we've gotten pretty much correct. You state you're trying to build a tunnel of, for example, 3 green cards. You flip over a green card, a red card, and a locomotive from the deck. We've assumed that you can now build your tunnel by adding two cards in any combination of green or locomotive to the initial three greens (i.e., you don't have to match the quality of the cards you flipped, just the quantity).
However, our confusion over ferries possibly creates a new question. If I had only 4 green cards (and no locos) in my hand... can I pay one extra green and three cards of any colour, in place of a locomotive to build my tunnel? I'm inclined to think not, but I just don't know for sure any more!