I think what Bill is remembering is a rectifier diode, probably something like a 1N4001. That would give fairly rough low-voltage half-wave DC; and the diode's current rating (1 ampere) is probably marginal. A better solution would be a bridge rectifier, with about a 4-ampere rating, like this:
http://www.radioshack.com/product/index.jsp?productId=2062580
This would also be easier on the transformer. Neither of these alternatives would give you directional control if used in the locomotive. I hesitate to recommend either of them, since I don't know anything about the motors that Lionel used.
Bob Nelson
Radio shack use to carry a in-line circuit about 1/8 inch around about 3/4 inch long with a firm wire coming out each end you could either hook up in the engine from you + side I believe befor it went to any electronics or the engine or you could hook it up prior to the track right off the transformer and basically it switched the power from ac to dc and you could run the engine with your ac transformer.
Bob Nelson may have better ways of doing it or more info on this as I remember a Friend in FL use to do it. don't even know the name of the circuit
Life's hard, even harder if your stupid John Wayne
http://rtssite.shutterfly.com/
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