I know this period of production well, and can say that there wasn't a factory-produced set during that time (or even close to it) that consisted of a steam loco, tender, boxcar and caboose.
There are two limiting factors given the information you've shared:
1) I don't believe there are ANY sets in that time period that only have a single freight car.
2) Even for sets that have two freight cars in them (assuming you're forgetting about one), one of them was very rarely a boxcar. In fact, I think the only times this occurred is for the sets made with boxcars produced for Toys R Us and True Value Hardware in 1976-77.
Without more concrete information, that's about all the help I can provide.
TRW