Cargo ships are attached to fleets in military operations and used for the transport of supplies.
The Giant-class was in use by 2491. It is capable of transporting a vast amount of material.
The Hercules-class was in use by 2504. During the Second Great War, Raynor's Raiders were able to retrofit them to create Hercules dropships.
Known Cargo Ships
Cargo ships were originally in-game units in StarCraft. Its wireframe can still be found in StarEdit.