Lionel 8902 and 8903 engines

They are in fact DC locomotives. Lionel made some for a while.

You could power them with DC, either from a purpose-built DC controller. Or add a bridge rectifier to your transformer. (See the recent directronic rectifier topic: ) But it might be more satisfactory to install rectifiers inside the locomotives. A 4- or 6-ampere bridge should be plenty big. (Unless you add an e-unit also, they will then run in one direction only.)