Trailers are stationary objects found at various points around Venezuala in Mercenaries 2: World in Flames.
There are at least three types of trailers in Venezuela.
Fuel tank trailers are commonly seen at the refinery, near UP semi-tractors. They will ignite and explode once shot a few times with small arms.
Flatbed trailers are extremely common, and require several rockets or explosives to destroy.
Cargo trailers are similarly tough, and are almost exclusively found at the refinery.
Note that while semi-tractors such as the Ogre Semi and Emissary Semi exist in Mercenaries 2, they may not be connected to trailers. This, unfortunately, deprives the player of the extremely entertaining possibility of a truck bomb propelled by a fuel trailer.