The full Eurostar route from London to Paris would be awesome! It's long, but would be awesome if it's possible.
The only limits are the TSW team and their corporate overlords. If they don't get the money to make a 150 mile route, they won't make a 150 mile route :P But I figure things will get better and TSW will get longer routes with extensions to existing routes, better building techniques that make routes longer but not more expensive, and maybe some more money from corporate DTG...but who knows if the business side even worries for the game side. Regardless, the eurostar line is a common requested idea. I like the concept as well.