Michael

They were marketed as 89'2" flat cars equipped for farm machinery loading.
Railroad rates for "loose cars" are almost always based on the size of the
car (and union work rules were often based on "cars") so by operating
"articulated" pairs of cars the GN could offer better rates than by simply
operating them as two cars. There were 63 of these "cars" on the roster in
1972. Other car types operated this way included 70 ton airslides.

I don't think that this existed before 1961, so it's after the STMFC era.

Tim O'Connor