as well as building a/c,i also do HO scale trains.inside the loco is a decoder.said decoder will fit the engine stated on the package.nmra(national model railroad association) states that the decoder will fix in loco with no problems.i dont know how many time i have read how the modler has sanded and filed just to make the resin part fit.i think its time for the resin makers to step up and cast the part with no sanding or filing.