This had never even crossed my mind as an issue before, but someone brought it up in a comment on another TTR-related question, and having checked the USA rulebook I'm no longer 100% sure:
When calculating who gets the "Longest Route" bonus card, do you go by number of train cars in the route, or number of links/cities in the route?
I suspect the official answer will be well-known to anyone who plays a lot of Ticket To Ride online, but for those of us who don't... what's the correct interpretation?