"I was thinking that the bodies were die-cast brass (not pot metal) and not formed sheet brass, like 3rdRail's NYC Mercury locomotive and the Lionel Centipede,"
First of all, our die-cast zinc models are not "pot metal", at least in the pejorative sense that I believe you meant it, and without this material, hard as it is to work with when I'm scratch bashing, we would have either plastic steamers (actually that would be fine, see earlier "scratch bashing" ref) or about 10% of the variety we have had offered in the last 20+ years.
Secondly, and I'm not sure I'm reading you right, but the 3rdR Mercury (boiler) and all the Centipede are indeed die-cast zinc. I own both models.
The 3rdR Mercury is a hybrid as I recall - the boiler/cab are die-cast zinc; under that the loco is brass. Sort of an upside-down-hybrid per the Lionel ATSF Mikado.