Why don't I get the Longest Route card?
A. The longest route is the amount of roads/boats between one point and another. Side tracks do not count for the longest route!
B. What many people don't know or realise at the time is that a connection between a road and a boat can only count as a route when there is a town or city between the two of them. A boat connecting with a boat without a town or city does not connect your route and thus your longest route will be cut off.
P.S. This rule does not apply to the Cheops expansion.