Shippers with any significant amount of traffic kept themselves well informed about how various routes performed overall and knew exactly how they wanted their shipment to go. If they didn't know how to find the cheapest rate (most did, and the biggest companies hired rate clerks away from the railroads to get the benefit of their knowledge) they could use traffic consultants, referred to informally as rate sharks, to find the best rate. I don't think it's possible to find out how many shipments were routed by shippers rather than a railroad, but it seems likely that the great majority of shippers, particularly regulars, wanted to retain control by specifying the route themselves rather than leave it to the railroad's local freight agent. Interesting to speculate, though, who the CGW might have given the car to at Chicago, since their revenue would have been the same, regardless. All the Eastern railroads had sales offices in Minneapolis (and plenty of other cities), and if a sales rep for one of them made a practice of regularly calling on local freight agents in his territory, and maybe leaving a few cigars, a freight agent for one of the roads serving Minneapolis might reciprocate by giving that (Eastern) road any unrouted cars--always provided that there was a valid route to the destination concerned--or even talk up the advantages of using that road in discussions with shippers whose practice was to specify routing.