If you want to expand your empire, you will be limited in the Lionel engines you can run with DC power, as most run on AC power. You would have to disconnect any whistle or horn relays on AC powered locos, as they are triggered by DC.
It would be recommended that you take your loco to a Lionel service station or a local electronics repair person, and request that a full wave bridge rectifier be installed. This way the loco will run with any Lionel AC transformer. It would run in only one direction with no reverse. If you want reverse capability, an electronic reverse unit would need to be installed instead, such as this one. https://www.ebay.com/itm/Lionel-Parts-610-8719-103-electronic-E-unit-board/333725834824?hash=item4db39c1e48:g:U9kAAOSwsdtd-8kO