Jimr,
there are two bogie flats, they appear in only one other shot of the train, two other photos show both K
B locos together, so there could have been a lighter bridge West of Springfield or just maybe the Traffic Manager decided to use that excursion to move the flats up the line. As for the U
S flats, those are all steel, weighing in at around 14 Tonnes tare(from memory), so might be a little hard lifting them out while pulling the train behind as well.
If there is interest I could post film of these K
B's in action over this line.