My take on the use of the terms "Route" and "Session" as used in Trainz are as follows:-
Route - also called a "Layout", contains most of what you see when in Surveyor and Driver. Think of a Route as the "hardware". The Route consists of
- Route Data - textures, landforms, water and turffx/clutter. These are "painted" into the Route using a brush tool.
- Route Layers - individual assets, both visible and invisible, placed in the Route. All track and track objects (excluding some trackmarks and triggers) should be in this group. The contents of the Route Layers will appear in every Session created using the Route.
Session - contains the "properties" of the layout and, optionally, some assets. Think of a Session as the "software". The Session consists of
- Session Data - mostly "invisibles" such as commodities in industries, loads in consists, weather, time, rules, drivers and driver commands
- Session Layers - individual assets that will only appear in this Session. In TRS19 PE and Trainz Plus it is recommended that all consists be placed in a Session Layer.
In addition, there are also
Driver Sessions which are "games" (or "running simulations") saved when in Driver mode.
EDIT:
The comments by several posters above are correct. Using layers is a matter of discipline and workflow. In addition the lack of any clear onscreen indication in Surveyor showing which layer is the current "active layer" is a cause of much chaos and frustration amongst route creators.
My workflow consists of:-
Only loading and editing the Route (not loading or saving a Session) when working in the Route only. For example adding/editing track, Route Layer scenery, ground textures, landforms, clutter, etc. Be aware that some Route assets (e.g. industries, junction switches) will have their properties stored as Session Data in a Session so I always ignore any requests to save the Session after editing these assets.
Always loading and editing the Session (both the Route
and the Session) when working with the Session Data and Session Layers. This covers changes to industries, consists (I always place consists in a Session, not a Route), rules, drivers, driver commands, switch settings (where the switch direction will be set differently in a Session), environment, time and Session specific scenery assets.