Not referring to the Kato version short wheelbase version.
I believe Atlas changed from using the classic RS3 chassis to using a new chassis with the correct wheelbase for the RS11 - which is a foot/foot and a half - longer than that for the RS3.
From their website:
Features:
Full pilots
AccuMate® couplers
Golden-white LEDs
Separate handrails and side sill
Correct wheelbase
Available DCC Ready & DCC Equipped