The PRR class X37 were steel-sided, 40-foot box cars built by the Pennsylvania Railroad itself in 1939 and 1940. By 1950 there were still 994 of the original 998 cars in rotation. The car shown here was the first of this class. The box cars used 2D-F19 and 2D-F20 trucks. They had an interior height of 10'4", which was slightly taller than previously-produced box cars. (photo PRR #65400, source: PRR railfan.net web site)