The Colonist Transport is a massive APC left abandoned on the outer edges of the colony. Presumably a heavy haul vehicle for moving excavated material, it does come equipped with an auto-turret to deter threats. The transport is very strong and thickly armored, making it very resilient to most threats.
In the Easy Company Campaign civilians are escorted into the transport to protect them from the zombies. Later, the truck can be escorted during the Colonist Transport Escort mission as it moves to the Containment Facility to pick up the civilians rescued during the the Seal the Bio-Domes mission.
Outside of Easy Company the transport has no active role in any campaign.