A Trailer is a cargo carrier used to transport heavy or large amounts of cargo from one point to another. It is used by Optimus Prime.
Optimus Prime had to use the trailer to transport the DNGS. It was able to transform into a flatbed, which allowed the Autobots to move the device to a train. The pursuit of MECH agents and Vehicon fliers caused the trailer to detach and explode.
Optimus got himself an identical replacement, which he used it to haul around the prisoner Starscream.
- Optimus was never seen using his trailer in season 3. It's likely that the trailer got destroyed in the old base's destruction.