The PRR interchanged traffic with the B&O near Washington, PA. Most of the traffic was hoppers and gondolas loaded with coal. However, I assumed that the occasional box car would slip in too. The B&O "wagontop" box car, so named because of its curved top rather than the more traditional mostly flat roof, came in two versions, the M53 and the M53A. In 1950 there were 1,935 M53 (road numbers 380000-381999) and 991 M53A (road numbers 385000-385999) remaining.
The external difference between the two versions is apparently a row of rivets running length-wise between the ribs along the top rounded corners of the cars. These were built in 1937 and 1938. The flat doors with which these cars were initially equipped where replaced with the lighter and better functioning Youngstown doors (shown in the photo) starting in the 1940s through the 1950s. I wasn't sure whether or not B&O "wagontop" box cars appeared on the PRR's Chartiers Branch, but just assumed so. However, one day I received an e-mail from Gary Carmichael. He models the Waynesburg & Washington RR in S-scale, had a column in the "1:64 Modeling Guide" magazine, and is a retired railroader who ran on the modernday tracks which used to be the PRR Chartiers Branch. He indicated to me that these cars were indeed interchanged with the PRR at Tylerdale, PA (near Washington, PA) for delivery to the Greif Brothers, Co. in Washington, PA (which produced cardboard packaging materials) for delivery to their own plant in Eighty Four, PA (an unincorporated area where the famous "84 Lumber" stores originate). These cars were also used for materials to and from a tinplate mill in Washington, PA. Gary's recommendation was that I should consider adding more than just one of these cars to my roster! (photo: B&O #380601, from Steam Era Freight Cars web site, which is now gone)