Very nice engine, thanks for sharing.
Many other Märklin Compounds (and also the replica...) have two covers (= cut-out heavy, nickel plated brass tubes) attached to the cylinders (as if there were cross-head-guards for the piston rods) - yours does have these. My own compound doesn't have them either - now I wonder if any only took them off - or was there a series without them...!?!?