A platform trailer includes a neck reinforcement structure located in the neck region. The neck reinforcement structure includes a first beam reinforcement structure located adjacent an inner side of the first beam that is oriented toward the second beam, and a second beam reinforcement structure located adjacent an inner side of the second beam that is oriented toward the first beam. A plurality of internal cross members extend between and interconnect the first and second beam reinforcement structures. A fifth wheel plate is fixedly secured to and that extends between both the first and second beams. A kingpin is connected to the fifth wheel plate and is adapted to be engaged by an associated tractor fifth wheel. The neck reinforcement structure allows for use of aluminum alloy beams with a reduced height in the neck region of the trailer.