In computer game terms a Route is basically a 'map' or 'level'. It defines the world in which you will be driving your train, including the landscape, scenery, buildings, track and of course the trains themselves. Trainz comes with a powerful map creation tool, called Surveyor. Surveyor allows you to create and edit Route and Session assets quickly and easily.
Routes vs Sessions
The Trainz concepts of a route and a session are closely linked, but very different. Both of them are used to define a drivable world within Trainz. Routes form the basis of the world, including the ground, ground height, ground textures and water. Sessions extend the Route to add game rules, environmental data (weather, time, etc), and allow customisation of some objects within the route (junctions, industries). With the addition of the Surveyor layers concept both Routes and Sessions can contain scenery objects (buildings, trees, etc), splines (track, roads, fences, etc) and trains. A Route can have an unlimited number of Sessions associated with it, whereas a Session is intrinsically linked to one specific Route.