Run8 only became the day-consuming thing it did for me because of the MP. Unjamming the network after previous players left it in a proper old state was a lot of fun, as was cleaning up the receiving yards to make room. Actual driving figured in it less and less, not while I could have a humper running and be dispatching on the other screen at the same time.
With a good AI to at least run the stuff along the mainlines and set its own routes, that would go a ways toward recreating that busy feeling. However, it was both the unpredictability of other players (as in real life) and the fact that what you did both affected and mattered to the rest of the server that made it so compulsive.