Looks like BART will carry a battleship when finished Joe. IM sure they are careful when welding on the frame, Heating the frame can remove the temper from the steel. I know that sounds silly, but believe me, it can crystalize the steel and make it weaker.
It doesn't look like they put a rubber strip between the bed frame and the frame rails. I would spray that gap areas with primer, then try to seal it with spray can rubberized undercoating. Messy, but it will really deaden sound and prevent rust. Keep those pictures coming!!
onezman
http://onezman.tripod.com/onezmans.conversions