Years ago in the 90s, I purchased a couple of cheap Lionel train sets. One was Crayola train set #6-11813 and the other was Atlantic Coast Line #9-1687. I also had a couple of handcars---one with Mickey & Minnie, and the other with Mickey and Donald. When Christmas time came, didn't want to bother with anything intricate, so I decided to set up four simple loops around the tree. Three "O" gauge loops powered by AC, and one Large Scale loop powered by DC. I ran all of them for a couple hours or so each day until it was time to take the Christmas stuff down in January. It was then that I took a good look at the boxes for the two sets. Both had instructions for connecting the lockons to the DC terminals. ACK!!!
Years prior, I had set up an over and under, O27 gauge, figure-8 track powered with an AC transformer. Being a kid, I wondered if my Marx Union Pacific HO engine would run okay on 2 of the 3 rails. So I put the engine on the track and gave it power. The engine took off with the headlight burning brighter than I'd ever seen before. It was GREAT and it was COOL! It went around the track maybe one and a half times before it suddenly stopped with the headlight still on. I turned the power off and back on, but only the headlight would turn on. The wheels wouldn't turn. I took the shell off, and I found one of the wires going to the motor had burned up.
Now, the motors in the Lionel engines didn't burn up, but I don't know if it was just because I didn't run them as fast as the HO UP engine or not. Did I shorten the lives of the motors on the Crayola and Atlantic Coast line locomotives or are the motors built to be powered by either AC or DC? I've always wondered about that ever since.