I no longer have TRS19 installed. I just ran a few tests using Trainz Plus/TRS22 and can report the following.
- I loaded into Surveyor a route (no session) that I had previously downloaded from the DLS. I made a change in the Session Editor only (added a rule) and made no change to the route. I then selected Save (Ctrl+S) and was presented with the options of Overwrite the existing session (when you load a route without a session a default session is always created so this will save the session with the name "Default" with a time stamp) or Create a new session (where you will have to enter a new name).
- I loaded into Surveyor a route (no session) that I had previously downloaded from the DLS. I made a change to the route, I moved a tree, and made no change to the default session. I then selected Save (Ctrl+S) and was presented with just a single choice Save a new route and session (where you will have to enter a new name for the route and the session will be named "Default").
From your screenshot it looks very much like Point 2 above, a change has been made to the base route not just to the session.
This is and has always been normal behaviour. When you edit a route and/or session (including the default session as identified in point 1 above) that is not yours, you have to save it so that it will have your user ID in its KUID code. In other words you have to save it as a new route and/or session, a clone. You cannot overwrite the original creators work because you do not have the right to use her/his KUID.
When you save your edited version of their work, you will end up with two versions of the route/session. The original as installed from the DLS and your modified version. The only exception to this is with DLC routes and sessions that have been locked against modification and cloning - that is the choice of the creator, not N3V.
If it is a route that you have edited and cloned then you can safely, if you wish, delete the original route.
If it is a session that you have edited and cloned then you can safely, if you wish, delete the original session
BUT you cannot delete the original route as your new session needs that original route to operate. If you delete the original route then you will also delete your new session.