Hi John --
Welcome to Trainz.
Coming back to your original question, the route or layout is the physical representation of the landscape where you can run locomotives, freight cars and passenger cars. Basically, it's the track plus all the associated paraphernalia like switches, signals, ... with whatever scenic features (buildings, textures, vegetation, ... ) that the author decides to add.
The session places locomotives, freight cars and passenger cars on the route. It may include interactive industries configured to produce or consume commodities. It may include tasks that the player is required to do. It may have pop-up instructions and scoring, or it may just provide instructions of the tasks to be completed.
I've possibly done as many routes, layouts and sessions as anyone else. If you look at one of my postings, like this one, it might give you a better idea of the difference between routes and sessions:
https://forums.auran.com/trainz/sho...w-TRS19-route-The-Rivercide-Electric-Railroad
As pointed out above, these Forums contain a wealth of information, particularly when using the "Advanced Search" function, like this, for example:
Phil