To add to the advice above, there are some additional factors that will need to be considered.
Many scenery assets actually have their properties stored in the Session, even when the assets themselves are in the Route (i.e. in a Route layer). Editing the properties of these assets is considered to be a change to the Session. These include:-
- junction switches - the left/right switch direction is stored in the Session (but the switch default direction is stored in the Route)
- industries - the commodities and quantities loaded/unloaded are stored in the Session
- industry enabled wagons - the commodities and quantities carried are stored in the Session
- consist locations - this is stored in the Session (TRS19 Platinum Edition, Trainz Plus and TRS22 only)
- named route objects - while the name is stored in the Route, adding or changing the name is treated as a change to the Session
Editing any of the above in a Route will result in a change to the Session and will generate a request to save the Session. Obviously, if you have edited the properties of an industry or rolling stock vehicle that loads/unloads commodities, then you should save the Session - but give it a distinctive name that is
NOT "Default". For any of the other changes (e.g. changing a switch position) can be ignored unless you consider it important for the operation a Session. For example, switch positions may be important if you want to create a Session where the user can drive a particular loco out of a yard and onto a main line without having to manually set the switches or use an AI drive command that will set the switches for them.
In all my many years of using a wide range of software, Trainz is unique in that it allows you to create and save a "parent document" (i.e. a Route) and then create and save "child documents" (i.e. multiple Sessions) that contain only the variations to the "parent document". I know of no other documents created by any software package that does this - others may enlighten me here. In Word and Excel, for example, you can create and save a document but what if you want to make another document which has a few minor changes to a single paragraph or additional formulae? You cannot simply save the changes only as separate documents. You must either make a separate copy of the entire document (with the changes) or replace the original with the new version.
Prior to the appearance of Sessions in Trainz (TRS2004 or TRS2006? I forget which) if you wanted to create two different "aspects" (for want of a better term) of your route, one based on passenger operations and the other based on freight operations for example, then you had to create two complete and separate Routes that might differ only in their rolling stock assets.
Yes, the Session system can cause confusion and as Boleyd has pointed out, it does take some discipline. The advantages it offers, in my opinion, far outweighs its disadvantages.
My thoughts.