A trailer is a cargo carrier used to transport heavy or large amounts of cargo from one point to another. Although such a purpose is useful in and of itself to a Transformer, many other Transformers find ways to alter or modify them if their alternate mode is capable of pulling a trailer.
Optimus Prime stored Roller and his combat deck in his trailer, using them whenever the need arose. This included the battle to prevent the Decepticons from leaving Earth and returning to Cybertron. However, during the battle, Megatron destroyed the trailer. Not to be out done, Optimus Prime merely "pointed" at the Decepticon leader, causing him to somehow receive heavy fire.
Over the course of the war, Optimus Prime would use the trailer to contain extinct herbivores Elita One to Alpha Trion's lab and carry the Dinobots to Sub-Atlantica, even though Grimlock has at least a few feet on Prime . If anything, the trailer is roomy., transport
Robots in Disguise
Optimus Prime's trailer contains his drone units, which combine with the Autobot leader to form his super mode. The trailer itself is nothing special. Just a mini base thingy.
Optimus Prime used his trailer to combine with his body to form a super mode. Oddly enough, in vehicle mode, Prime's trailer held rather large cannons, despite the fact that it was a fire truck. Scattorshot brought this up, but Prime merely replied that a fire truck was larger than a tank.
Optimus Prime's alternate mode is technically a fire engine, meaning that he shouldn't have a trailer. However, Prime is capable of detaching from the engine into a smaller truck which is capable of pulling other trailers.
A young boy once asked where Prime's trailer went when he transformed into robot mode. Optimus was going to answer, but we don't know what he said.
Dark of the Moon
After two movies, Optimus Prime finally gets a trailer.
It is nearly identical to the appearance of his Generation 1 counterpart's trailer except for the addition of the blue refrigeration unit. The trailer transforms into a ring-shaped weapon rack. It is only seen to transform in the movie once, when the Driller rams into Prime, knocking the trailer free. The trailer is seen to hold several machine guns, a shield, an Energon sword, an Energon axe, and it is able to transform into Optimus Prime's flight pack.
- When Optimus Prime transforms into robot mode, his trailer usually disappears, only to reattach itself to Prime whenever he goes into vehicle mode. Many fan theories about this disappearance have arisen over the years, with the most popular being that Prime stores the trailer in subspace.
- In "Career Day", it is shown that Transformers Animated Optimus Prime himself doesn't understand where the trailer goes, or he has a hard time explaining it.
- Another short explains that his trailer is equipped with a homing function that allows it to locate Optimus and re-attach if necessary.
- In one of the early issues of the Marvel US G1 comic, Optimus Prime's trailer didn't disappear but rather just stood where Prime battled, waiting for him to transform back.