In addition to Mr. Hall's excellent description of lumber forwarding routing, there is also a provision on the waybill for "Agents" routing of a car. In these cases a shipper would simply specify a destination, and let the origin road route the car. The origin road would, of course, try to maximize their revenue. The shipper would pay the full freight to the origin road, who would then give a division to any other roads in the route. On "Shipper" routing, the shipper would specify the route, in order to pay the lowest overall rate. Agent routed waybills, on a loaded revenue move, are very very rare and may not even exist, except in theory.

As for the original question, gateways are important because they're points where roads are allowed to interchange. The bigger gateways, such as St. Louis or Chicago, would all have neutral switching service, so if you've got access to the gateway, it means you've got access to all the roads serving the gateway, even if you don't directly connect.