The PRR Class X-31 box cars had a novel roof design named the "round roof" or "wagon top." By curving the roof sheets to meet the eave bar vertically (leaving a 4" inset on the edge) the cars had an inside height of 10'. The X-31's were used primarily to transport new automobiles or auto parts. The majority of X-31's were first built in January 1933. In 1935, the roof of the X-31 was redesigned with a new inside height of 10' 5" in the center and 10' at the sides. The external appearance changed with the roof sheets overlapping the side sheets at the eaves. This gave a flush appearance to the roof as compared to the inset of the original design. By 1936, the single-door X-31A had replaced the X-29 as the general service box car. The X-31's were used on the Pennsy throughout the 1940s, 50s and the mid 60s.