From said help file:
"Trains, convoys, and barges are built into the terrain. Each train, convoy, or barge has a source point and a destination point, and each can be damaged or destroyed while traveling from point to point. When a train, convoy, or barge is destroyed, they will automatically respawn every ten minutes.
Each time a train, convoy or barge spawns, it first checks to see if there has been damage to the supplier (cities supply factories and factories supply fields). The amount of supplies carried to the destination depends on the amount of damage to the supplier. If the supplier has been damaged by 50%, only one out of every six train cars or convoy trucks will carry supplies.
The destination point is where the train, convoy, or barge attempts to resupply. The destination can be either a single field, or a number of objects within a defined radius. If a destination field is captured while a train, convoy, or barge is enroute, the train, convoy, or barge continues to the destination, and resupplies from the factories of the capturing country.
Once a train, convoy, or barge has successfully reached it's destination, a new supply convoy spawns."