I have developed an interest in the PRR mp54 cars hauled by steamers. I have read about combines and seen photos of mail/passenger commuter cars. My question is did these consists ever include the heavyweight 6 wheel trucked headend cars, such as BM70s and/or B70s? I read a post on another forum (site misplaced) that referred to a rule/directive that the p54 cars should not be placed between heavy steel cars or between the locomotive and a heavy steel car. Any thoughts?