I believe, like you said, that you would be more correct in calling it container/intermodal traffic (I don't even think that term had come into use yet, let alone "invented", but I could be wrong)
The Apollo/Mercury trains were the ones to carry this traffic (I am not a "late LV" historian/modeler, so please corrct me if I'm wrong), since this was, in the 1970's, starting to replace box car frieght, and especially in how frieght was handled at major ports, ESPECIALLY NY Harbor.
Did the other area rr's handle it? I think the EL did, and I want to say you might have seentraffic like that on the CNJ towards the end, like on trains ES-100/SE-99 to Scranton.
Keep in mind, tractor trailer on flat car traffic was NOT a new thing, going back all the way to the 1930's........However, things like the EL's UPS trains, and PC's marriage of PRR's Truk-Train and NYC's Flexi-Van services into TrailerVan (continued by CR), were probably stepping stones to containers, and then stacks.
As a side note, I'll never forget my first "Stack Pack" experience - It was on a rainy Sunday in Port Jervis behind a NYS&W C-430 and BN-green SD-45....OK, not LV related, but a fond memory none the less.
Ralph AKA CF
M.P. 28.5 On The NY Division