The American Flyer transformer will work as both Lionel and A.F. use A.C. voltage. The A.F. may only go up to 16 to 18 volts, depending on model, verses the Lionel transformer going up to 21.5 volts with the post-war ZW.
With a Williams engine, who wants a higher voltage?
Philadelphia & Reading Railway, one of the first railroads in the USA, first to have a double track system in the USA.