I like to try and keep things simple when in Trainz.
Re screenshot:
The "Main session" is where I add the Driver commands, rules, track, road and track vehicles etc via "Edit session" and get's saved as "Save session".
Default layer for this session is Session. It is also used to "Drive" in.
When the thumbnail is hovered over this session, after a small time it will show the session's name, date and time.
The "Driving saved session" is where I have been driving in the Main session and made a save. The driving save shows the visible name of the session, date and time. When the thumbnail is hovered over it will show the session name, date and time also. When this save is reloaded for another drive, the trains will continue from the point they had moved too. I definitely do not edit this save as it is not one of the route's main sessions I work in.
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My basic Golden rules for world building:
When making the route world, it is worked on via "Edit route". The default layer is "Route".
Saves are "Save route" and "not the session".
When adding a track/road vehicle, driver commands/rules, it is done via "Edit session". The default layer is "session".
The session save window will show the session's custom name only.
Of course a locomotoive that is in the session layer will not be seen when working back in the base route itself.
It only takes a moment to jump between "Edit route" and "Edit session".
I also use custom layers within the Route and session layers, and the above rules apply when saving.
Example: locos will go in their own layer in the layer tab under "Session > locos". The "locos" layer is highlighted when the locos are added to the session. The loco layer is then locked so no other content can be accidentally added to it until wanted. Also if wanting only a particular type of trees/plants etc showing only in that session and not in the route or other sessions, a custom layer in the session layer tab is made e.g. "plants". This "plant" layer is highlighted so when the plants are added to the session, they will be in that layer only. The layer is then locked. Can have a lot of fun with layers.
UDS, Unified Driver Surveyor:
Most of world building is via "Edit route" but when one comes across something minor that needs to be changed when Driving a session, UDS is great for this.
However, do need to be careful how the saves are done.
1. When "driving" and then returning to surveyor using "Surveyor Mode" (to add/move/remove a tree for example) , the defaulted layer is still "Session" in the layer's tab (right side). The session layer needs to be changed to the "Route" layer before the tree is added. The save is "Route only" and "not session", when returning to Drive. If the tree is just moved, the save is "Route only" and "not session" also.
2. If the session is saved too the vehicles will be at the point they have moved to, so when the "Main" session is driven at another time, the trains will continue from where they left off and not at their default starting point.
3.When content has been amended/added in the Route's layer "surveyor mode" via "Unified driver" double check that the save is only the Route option in the Save's window and select "Do not save session".
4. Make frequent cdp backups of route and session/s via Content Manager when everything is working as it should be. My backups have saved the day quite a number of times - especially when experimenting and learning with using custom layers and UDS.