Magical portals that link the various Kingdoms.
Each Portal can transport a set amount of mass-per-second based on size. Obviously, the Gate can handle a transit of mass-per-second equal to its capacity indefinitely. Exceeding this capacity causes the Gate to 'charge' until such time as the MPS balance has been restored.
The capacity of a given gate is one hundreth of a pound per square inch. So, for example, a 96"x96" gate could transmit 92 pounds per second. The 'standard' gate size is 96" x 24"; or 23 pounds per second. The major cities of the Kingdoms contain far more massive gates used to transmit entire caravans of wagons or columns of troops at a rapid pace.
The Gates cannot be active all the time; each gate can perhaps be active one hour in every four or five. Some gates have been constructed for more or less downtime depending on the skill of the Wizard and artificers who constructed the gates.