I realize that OP Arnold is primarily interested in the northeast. But out west in the pre-Amtrak years, Union Pacific city trains would often have Pullman sleepers in their passenger train consists from other railroads.
I rode on UP trains to California from Idaho when I was in high school back in the 1960s (yes, I’m THAT old 
). When connecting in Salt Lake City, I remember seeing Pullmans from other railroads on trains I took to Los Angeles. They were painted in Union Pacific livery, but had the railroad’s name on them, like Wabash and The Milwaukee Road.
I guess these were Pullmans from midwestern railroads that hooked up to UP trains in places like Omaha for thru-service to California.
