The 531 Hauler is equipped with a tailgate that can be opened or closed like the Val's All-Purpose Hauler. Unlike the Small Trailer, the 531 Hauler features four wheels, and a much bigger cargo capacity. This trailer is also the only vehicle in game that currently features an advanced suspension system. If you look closely, you can see the wheels rotating themselves to an angle perpendicular to the ground, which provides suspension.
This trailer can be spawned on its spawn pad for $260 Money, and can appear in many different colors, and in very rare instances, in hot pink.
It is possible that the name of the trailer is a 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 double of the proposed cost.
The 531 Hauler's spawn pad is 24 studs long, 12 studs wide, and 0.2 studs tall. Strangely, in the past, the spawn pad used to be 14 studs wide. The 531 Hauler itself is 21 × 11.8 × 4.4 studs if the tailgate is raised.