In 1934 the Pennsylvania Railroad started production of a new series of 40' boxcars the X31 series. These cars featured an unusual round roof appearance to provide maximum capacity within the then-standard loading gauge. The most numerous group of these new boxcars was the X31A, produced in both single and double door variants. In total, the PRR produced 6700 X31A boxcars, with another 2000 cars being produced to similar classes X31B and X31C. This made the X31 series the second most numerous series of PRR boxcar after 1949.
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