You're welcome Carel. This is one of those gotchas we've all fallen for. A very long time ago, I made a post called "Will the real Default session, please stand up?", or something like that.
N3V took note and finally after a long time has finally added the option to save route-only when editing. This will allow you to save your route as you edit and save frequently without creating too many default sessions.
Once you've "finished" route editing, you can then use the default session as a base to create a driving session from. What we, or I am referring to, is the session where you set up all your drivers and configure your industries and stations and anything else that may need setting up. I usually call my session Daily operations and then use that to create other sessions such as a freight-only operations on a mostly passenger one, or maintenance day where I have the ballast trains out on the mainline. These separate driving sessions, as you can see, allow you to make many variations on a theme.
This will all come to you in due time and with experience.