I believe that the two most used multiplayer routes are the UMR and possibly Montana Rail Link. Their corresponding sessions have almost the same name. I almsot forgot to mention, you will need to download the latest versions of these routes which are "UMR-2013-Summer-Multiplayer" "UMR-2013-Summer-Multiplayer session V3.1" and "Montana Rail Link 12 MP v2.8" "Montana Rail Link 12 MP Session v2.8." Beware though on the Montana Rail Link, because there are actually sessions/routes with that name. Fortunately when you download the incorrect one, it updates to the correct version.
Some other good MP routes include:
Port Ogden and Northern by Colourlight
USA Prototype Route by Danny5
Thompson and Western by GN1996
Those are all I can think of off-hand, but I am sure there are many more. And always remember to download the latest version on any route, otherwise you may find yourself waiting for a session to appear for a very long time, as people tend to use the latest version.
I hope you have lots of fun in the multiplayer world!
GN1996