I've tried to step up "infinate length" sessions. It is rather difficult if you use a larger (>50 miles, >100 km) routes. I use a combination of staging yard(s) and portals. I use the portals to generate "generic trains" to supply cars to online industries, consume unneeded 'spare' trains, and to load-unload trains for said industries. The staging yard holds through-traffic that runs across your layout, more for senic affect.
My attempt was to step up a 'persisant railroad' on HP's Marias Pass (my computer was a limiting factor). I had Amtrak, some BNSF Mixed Freight, and some BNSF intermodal traffic in the staging yards acting as through traffic. My portal-generated "generic" train included loaded cars for the Aluminum Plant on the route, some empty flatcars for the foresty industries, some empty tank cars for a refinery, and some empty boxcars for goods produced along the line. My downfall is that the route, though beautiful and very well done, was too big. There was too much to keep track of.
I'm still trying to set up a persistant railroad (that really is a good term for this kind of thing- bully for you!); I'm building a route especially for the purpose. My advice, choose a shorter length route with a fewer amount of online industries, at least untill you get the hang of it. Also choose operations that use shorter-length trains: narrow-guage, branchlines, local/wayfreights, passenger operations, etc. I tried to use >100 car trains on my attempt (prototypical of the real Marias Pass). My computer suffered because of this.
I'm glad I'm not the only trainzer crazy enough to try this...
