Route and Session asset types are the framework for the maps within Trainz. This page builds off the concept of routes, so it is highly recommended you read the Route documentation on this wiki before reading about sessions.
What Is a Session?
A session can be thought of as a customisation of a route that does not alter the route itself. Specifically a typical session defines the rules of a game and things such as train starting positions, initial junction directions, and certain industry setup. With the addition of Surveyor layers in TS2010 it is now also possible to place any map object, track/road spline or train in either the route or the session. This can allow extensive customisation of a route without actually editing the route data.
How Do I Create a Session?
Sessions are created within the Trainz Surveyor module and can be kept simple, or be made very complex. It is not uncommon for a well structured session to contain dozens of custom rules but some may only use the default rules. For an in depth explanation on how to create a session check out this how-to guide. An introduction to session rule theory can be found here.
TBD: more details on layers, including a link to the surveyor layers page. TBD: more detail on why you would create a session, and what should go into the session vs route. TBD: information on the ability to white-out content using layers.