Complex sentence: Some railroads purchased semi-permanently coupled A-B-A three-unit sets of, while a few, like the Santa Fe, ordered all their FTs with regular couplers on both ends of each unit for added flexibility.
Split into simpler sentences: Some railroads purchased semi-permanently coupled A-B-A three-unit sets of. Some roads, like the initial customer Santa Fe, ordered all their FTs with regular couplers on both ends of each unit for added flexibility.