Simple history lesson. Lionel trains used AC motors until the 1990's. And perhaps a bit later for some of them. For example, the Platinum Ghost model from about 2000 showcased open frame motors with a clear shell so you could see them in operation. If they had abruptly switched from AC current to DC, all existing trains would have been incompatible with the newer ones. By converting AC to DC in the engines, they all coexist. That way you can run a Lionel from the 1950's with a current engine. Try it sometimes. The old ones have their own magic.