I'm sure there are other great ones, but I really like the routes by gfisher. They're realistic, nicely detailed, and fun to operate. I haven't had a chance to play with all of them, but my favorites so far are the Bridgeport Southern and the Cattaraugus Creek & Lake Erie.
Also, at the risk of sounding immodest, I've uploaded a few US routes myself that you might enjoy.
Have fun,
Max