The F-M opposed - piston engines used 2 sets of pistons, rods, and upper & lower crankshafts, but only one cylinder for each set of pistons. The 2 pistons in each cylinder compressed towards each other in the center of the cylinder. I recall the exhaust was scavenged from 2 opposite sides of the block; however the mufflers and exhaust stacks were normally on one end of the block, while the main generator was on the opposite end ( usually the end closest to the locomotive cab).