At specific checkpoints, Trading Post sites can be built to gather experience (or, if the Trade Route is upgraded to accommodate a stagecoach or a train, the other three resources too), to which the trader will provide resources to the player that controls these, each time he travels through.
The amount of resources varies greatly provided depends on the size of a map, but also on the number of checkpoints. If a player manages to control most of the Trading Post sites, including these found on Native settlements, the trade monopoly victory condition may be activated.
Two upgrades are available for the Trade Route: Trade Cart, and Iron Horse, both of which upgrade the speed at which you gather resources. They also allows you to choose food, wood, or coin from the Trading Posts, rather than experience.
Note that upgrading the Trade Route will increase the gather rate for ALL players for that Trade Route, not just the player or their allies.
Resources will be wasted if both player and ally upgrade the Trade Route from their own Trading Posts at the same time.