The 531 Hauler is a trailer that can be bought at Boxed Cars for $13,000 in-game Money. It is the larger of the two trailers.
It features a tailgate that can be opened or closed like the Val's All-Purpose Hauler. It has four wheels unlike the Small Trailer, which only has two wheels. This trailer has the largest capacity among all trailers currently featured in the game. This trailer is the only one that currently features an advanced suspension system. It is also the only vehicle that does not have wheels under the bed. To respawn a trailer the player will need a Vehicle Spawner and $260. This trailer is a really beefy trailer, if you look, the wheels actually moved to the angle of the ground, providing suspension. Unlike the normal trailer, this thing can carry a ton. Just like any other trailer, it comes in many colors, including pink.
It is possible that the name of the trailer is in reference to the user Spacek531, who proposed the idea of the trailer to Defaultio in a tweet. The cost of the trailer in the game is exactly twice that of the proposed cost.
View of a bogie exposed by a glitched trailer.
Dimensions
The Hauler spawner with a Robloxian to scale
The 531 Hauler next to a Robloxian to scale
The 531 Hauler spawner is 24 studs long, 12 studs wide, and 0.2 studs tall. 24×12×0.2 (Although at one point the spawn pad was actually 14 studs wide).
The 531 Hauler itself is 21×11.8×4.4 studs, with the tailgate closed.
The holding area is 14.4×8×2 studs.
It is safe to assume that opening the tailgate then raises the length to 16.4 studs.