Trailer

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.

If you're the leader of the Autobots, it's pretty much a rule that you have to have a trailer. There are a few exceptions.

Generation 1

• Optimus Prime has a trailer that stored Roller and transforms into the combat deck.
• After being rebuilt as a Powermaster, Optimus Prime was able to combine with his trailer to form a super mode for himself.
• Action Master Optimus Prime didn't have a trailer, but his vehicle, Armored Convoy, did.
• Ultra Magnus combines with his car-carrier trailer to form a super mode.
• Motormaster transforms into robot mode with his cab and trailer as a complete unit.

Classics

• Bumblebee and Cliffjumper tow the Wave Crusher that transforms into their jetpacks.
• Bug Bite has a similar trailer, Tidal Wave, with slightly different abilities.

Fiction

Generation 1

Ultra Magnus's super mode was generally treated as his default robot mode to distinguish him from Optimus Prime.

Dreamwave comics continuity

As the exception to the rule, Ultra Magnus shed his super mode armor to survive a blast from Shockwave. War and Peace

IDW Comics

Optimus Prime is bounced down to Brasnya without his trailer. Several panels later, his trailer arrives via the bounce as well. Prime later downloaded his consciousness into secondary buffers in the trailer section when attacked by Megatron in Brasnya. From his description of the process, it appears that he may have done this before, but not at a distance or under combat conditions. His trailer stores Roller and transforms into the combat deck. Escalation

Generation 1 cartoon

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. More Than Meets the Eye, Part 3

Over the course of the war, Optimus Prime would use the trailer to contain extinct herbivores Dinobot Island, Part 2, transport Elita One to Alpha Trion's lab The Search for Alpha Trion and carry the Dinobots to Sub-Atlantica, even though Grimlock has at least a few feet on Prime Atlantis, Arise!. If anything, the trailer is roomy.

Robots in Disguise

Optimus Prime is capable of combining with his trailer to form a super mode, while Ultra Magnus combines with his trailer to form his, uh... Legs.

Unicron Trilogy

Armada

Optimus Prime is capable of combining with his trailer to form a super mode, making him taller than Megatron at least.

His Minicon Sparkplug is able to drive the trailer, called the "container" by sitting atop of it in vehicle mode. Jungle

After being attacked by Decpeticons, Optimus Prime fired his weapons from the trailer in defense. After retaliation by Starscream left Optimus dazed, a bunch of Mini-Cons manned the weaponry. They continued to fire against the Decepticons and were eventually successful in causing Cyclonus to lose control and drop a Mini-Con panel. Prehistory Sparkplug appeared out of nowhere at Optimus Primes beckon to help combine him into super mode to fight the Decepticons. Conspiracy Again in his super mode, Optimus un-combined with correct timing, causing his trailer to fall onto Megatron. Trust

Energon

Optimus Prime's trailer contains his drone units, which combine with the Autobot leader to form his super mode. The trailer itself just tilts onto its end, forming a garage/launch bay for the drones.

Cybertron

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. Inferno

Animated

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. Transform and Roll Out! Headmaster Garbage In, Garbage Out

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. Career Day

Trivia

• 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. IDW comics gives a nod to this, by having the trailer arrive at Prime's location using the orbital jump.

• Flint Dille's explanation for Prime's disappearing trailer is the substance, Disappearium.