Just to enlarge. In the past a route automatically saved a "Quick Drive" session. You could go to "Route" and then "Session," and "Quick Drive", place your loco (or whatever) and do what you wanted. But you were only in "Driver."
Recent versions of TRS19 have the UDS as mentioned above. No "Quick Drive" is automatically generated for a route. Instead, you "Create Session" and then when in a session you do the same as "Quick Drive." But the advantage to this new system is that once you place a loco (or whatever) you can click "Tools" (wrench and screwdriver icon) and go into Driver mode to drive. This going back and forth between Driver and Surveyor (used to build routes) in UDS is much faster than the old system.
You'll be reading a lot more about UDS in the forums. Some users like it. Some users don't like it, especially some of the new menu changes.
Hope this info helps and provides a bit of background.