Carl Tuveson has documented a factory error in the construction of the power trucks on the GP engines, and an easy solution. The problem is detailed here - more information is available at the S-List Files Section.
Most of the Lionel-American Flyer Geep and EP-5 series engines have what I consider a defect in the assembly of the power truck. The wheel-axle assemblies of the power truck are switched front to rear. The traction tire wheel should be next to the truck side-frame power pickup. Not all of the trucks were assembled with this defect, but most were. The only one I have that was correctly built from the factory was the 370 GM demonstrator re-issue. By switching the axles from front to rear in the power truck frame it will spread the trucks ability to pickup power over switch frogs and crossovers and greatly improve performance.
(photos by Carl Tuveson)
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