Trainz Driver Edition allows you to only use default content in set routes and sessions. Trainz Driver has more sessions for Marias Pass Route, and more of it. Plus, some of the other routes are included, along with different content. (Amtrak F40PH free-edition, BN GP38 free-edition, and some cargo) You can view the content, and that's it. Not much.
What TRS2006 lets you do, is have some less Marias Pass Sessions and length, but more routes. Not to mention access to all content TRS2006 and below through Content Manager. It will download content from the DLS (only if you have a First Class Ticket, since it going to be unsupported), allow third party commiting of content, and allows you to make your own stuff! Plus, you can edit and create routes and whatnot. It's truly a Trainz simulator. You can purchase Marias Pass on HP Trainz's website (Marias Pass Route Plus) with the F40PHs Payware-edition. (custom cab, intense scripts, and many wonders to boggle your mind) In addition, a better-looking route in general, and BN GP38. (Keep in mind it's an upgrade, so it will make Marias Pass Approach look much better.)
Cheers,
Joshua