Technically there are few engines that run on DC track. They were made in the MPC era. One is a 8161 switcher. You have two options. GO to the Lionel Supplements and save the 1-9 supplement. This is the MPC era and the index will tell you what engines us DC.
Common sense is you second option. Engines before 1970 are AC. A DC motor is a can with magnets. An engine with only a can is DC. Now with wonderful electronics, an electronic e unit is added to allow the DC motor to run on the AC track.
That is the difference. Old AC engines can run on DC BUT! the whistle will run continuously. I am not sure if changing directions is doable, probably not.
This page under product supplements click 1-9. Page vi lists some of the DC only engines.