It really depends on what you want to have in a sim. Run 8 is geared toward running a train realistically as possible and the scenery is improved all the time but it is not their main focus. Train physics is. You most likely, especially on the West Coast routes, will not spend much time looking at the scenery, you will be looking at your MFD screens or gauges trying to keep your train under control.
You don't need to purchase extra rolling stock to operate the sim but you will see a lot of Run8 Western box cars if you don't. I don't recommend purchasing the extra rolling stock though until you decide if you like the sim. It's hard to beat for train physics. I would check it out and then start purchasing the rolling stock a bit at a time if you like it.
As far as AI, right now you do have to dispatch it but you can control how many other trains will spawn from the spawn points so you can set it up so that you don't have to spend much time watching the DS board. When in single player you will have to visit the DS board anyway to line your own route. For doing so more easily, go to The Depot and download the stand alone External Dispatcher. It's only for the West Coast routes at this time but it's free and easy to use and makes it simple to set a route in a few clicks. If you go that route and have questions about how to set it up you can PM me.
The V2 initial purchase will get you from Bakersfield to Needles but you would have to get the Cajon route to get down to West Colton and San Bernardino. Coming soon will be the LA route from West Colton to LA and the Seligman sub which goes east from Needles. Running from LA to the end of the Seligman sub would be quite a long day.
I hope this helps. I'm sure others will chime in as well.
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