To clarify: Some of the time, some of the entire trains to and from the South were hauled by GG-1's over the PRR between Washington Union Station and New York Penn Station without the addition of any local PRR cars or passengers. South of Washington, other railroad's engines were used, and sometimes the other roads added other cars there. The most likely examples would be the highest class, often extra fare trains, like the Champions, some of the Silver trains, and the Crescent Limited. Lesser trains were probably combined with PRR cars, trains, and passengers.
I'm not sure how to tell the difference between a foreign train with PRR cars added North of Washington, a PRR train with foreign cars added, a foreign train that has some PRR cars in it that stay in the train South of Washington, and any of them that also allow PRR local passengers to ride.