Now i've only had this happen once, and that was after doing a bunch of Undo functions to fix something. In hindsight, I would have been better off just closing the route and starting over on my edits. Perhaps I've been lucky and should go off and play MassMillions!
So to prevent the multiple Default Sessions, which is really confusing, I've come up with a restricted saving routine. When I save a route, I do the Save, and do not save a session. Once I'm done with my route editing, I will then go into the Sessions menu, in the main launcher menu, and create my session there. I will then save a session as needed, and sometimes will end up saving the route there too if I need to edit a portal or something, which is when this is required.
Now, should I need to edit the route again, I will not make any kind of edits in the Session side of things other than placing rolling stock, configuring portals, and setting up rules. The route editing is again done only from the route menu. This is a bit more work, but it keeps the two functions separate.
I agree the way N3V has setup the layers along with the route/session saving in Surveyor is confusing. There is no reason for this confusion and creation of mutiple default sessions with a save. This is why I came up with my restricted routine of doing route edits only in the route editing side of things on the menu, and session edits on from the sessions menu.
John