I was able to put the Atlas/Kato and Atlas/China shells on the Bachmann RS3 with no body or frame work needed.
Just drop it on.
The only thing I had to do was add a coupler shim , as the coupler came out a bit high on the Bachmann frame.
Putting the Bachmann shell on the Atlas frame will require work around the cab and battery box.
I didn't try it, but it doesn't look to be that big a deal.
MIchael Pennie