In the latest versions of Trainz the draw distance is govenred by a 'Limiter' which automatically reduces the draw distance to try to match the graphics load with the computers resources. The route's size is irrelevant, the issue is the amount of content in the current view. It is quite possible to simply rotate the camera away from a hi-content area (a big yard or a city scene) to a lo-content area and watch the draw distance increase as the load diminishes.
In older versions of Trainz the draw distance was absolute - if the load got too much: boom - hello desktop!
Andy