1. Hub and Spoke. (https://en.wikipedia.org/wiki/Spoke%E2%80%93hub_distribution...)
2. Optimized traveling salesman looks very suboptimal locally.
----
For more details: try playing OpenTTD with cargodist (OpenTTD 1.4.0 and above, roughly June 2013 or later). This is where mail / passengers / cargo will automatically route themselves and plan where to go rather than the default pathing algorithm.
You'll find that the most efficient route is to fill up your trains such that they're full. __Not__ necessarily for individual packages to take the shortest path. In fact, many routes are locally suboptimal (ie: moving away from the destination) so that you can reach a hub.
Sending a train with 100% of cargo down your rail-lines is far more efficient than sending two trains with 50% cargo. Similar issues happen to airplanes, busses, trains, and ships in real life.