My first O "scale" (not O gauge ie Lionel) was one of those F units. That purchase (at a train show in the early '80s) opened the floodgates to my getting involved in 1:48 size.
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I think the basic platform is very..."do-able"; I have acquired several more and on each I have altered either the shell, drive train or both, just because.
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Atlas must have sold a bunch, as there are almost always some for sale on eBay...
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Come to think of it, maybe they could be considered the "Enhorning of O scale"; a good enough product that just didn't quite make the cut.
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Mark in Oregon
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Mark Mugnai