For what it is worth, here is my Rookie method:
Almost all routes are immediately cloned after download. Name for the clone always includes the start of the original title and something to signify edit.
For example, you download "Bridge&Tunnel RR." You would make a copy and name it, "Bridge&Tunnel John2002." In the Edit Route dialog, you can keep the original description and add notes as you edit.
Every Route wants a Session, so saving the clone will produce one. It is a good idea to rename it from Default. In Surveyor, it will say e.g., "Bridge&Tunnel John2002 - Default," but in other lists it would show only as Default. Much easier to work with if renamed. For example, "Bridge&Tunnel John2002 Edit Session." Then delete the one named Default.
Any editing done after that, use the Edit Session. Make changes to Route, Session, etc., and then Save: Overwrite Route and Overwrite Session.
I only use the one 'Edit' Session until Route edits are complete. Any new, variety Sessions would only edit Session-specific things like Environment,Season, Consists, Industry configuration, rules.
As I said, one Rookie's method. YMMV.
Regards,
Blabsky