Hi All and you too Jay ,
Yes, it's a fairly tight fit, but if you pick the loco up by the shell (I know, generally a no-no) it will pop right off in your hand and the frame, motor, trucks will drop out. I'll look at it better tonight as I forget exactly how RC did it, but IIRC, the RC plastic sub-frame allowed a couple of screws to hold the shell, walkway and sub-frame together.
Kelly's frame replaces the RC sub-frame and the old type of installation is not possible, so we have to devise a new way of doing it. I lost Kelly's email address so I can't ask him if he came up with a fix.
I just didn't want to re-invent the wheel if someone came up with a way to securely attach it together.
bh