The PRR did not have "wagontop" boxcars. They did have round roof boxcars, which were of a different design. PRR's round roof cars were flat roof boxcars where the roof was replaced with a rounded one for more internal volume, but still have clearance for PRR's low tunnels.
I believe the B&O's wagontop cars were specifically built that way to limit water leakage at the roof line. I don't have either type of car, so it would be interesting if someone posted a picture of the two together to see the difference.