ok, so you pay six to build the ferry? Does it cost any extra time to ride the ferry? (I think I was still confused about this when I had the rules in front of me.) I don't have Empire Builder, but I do have Iron Dragon so the other points I can figure out.
Yes, you pay 6 plus the cost of the milepost you build to out of the ferry. When you train reaches one end of the ferry it ends it's movement for the turn, then on the next turn it moves out the other end at half rate (5 spaces for a 9 train and 6 for a 12). The other EB games with ferries are Eurorails, Russian Rails, Nippon Rails and India Rails.