That is what I said in my response, however it sounds like Big Daddy might have something different than the KATO drive Stewart F units I am used to seeing.
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He said something about it looking like an Athearn drive.
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It could have a Proto-Power west chassis or maybe like the later Stewart Baldwin switchers, it might have a modified Athearn drive.
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I cannot access my Kato/Stewart F units right now for pictures.
I have Stewart branded F7’s and FTs, and neither one has Kato drive. My F7s have a Buhler motor. Don’t remember what the FTs have. I bought one of the power chassis and swapped one of the A unit shells, but on the B unit, since there is so much space in thre, and everything is in place to pick up power, I installed a sound decoder and a BIG speaker. 2 powered As and the dummy B can haul any train I need, so I felt no need to power the B. My FTs are all powered.
Just to emphasize, you should be able to use any Stewart F3 through F9 powered chassis but NOT the FT. It’s a different design; as with the real FT’s the trucks are not evenly spaced. The rear truck is close to the back of the chassis, while the front truck is set back.