phrankenstign, either your memory is faulty or those two Lionel engines had been previously modified with an in line rectifier. Both of those engines as made were DC only and came in sets with a DC power pack. If you had put them on track with AC power, they would not have run at all and the longer you left them on the track with AC current, the more likely you would have ruined them.
The vast majority of trains made with DC motors (and there are many) have a rectifier in the reverse unit circuit board, which allows those engines to run successfully on either AC or DC current. Although some engines with DC motors that have other features, such as sound, may not be able to run on DC current without causing damage to the sound boards. Lionel locos with "Trainsounds" are an example where the instructions warn not to run the loco on DC even though it has DC can motors. That's a case for paying attention to the instructions that came with the loco.
There are many who consider the DC only engines by Lionel to be a mistake. Of course, this was an intentional cost saving avenue so the retail price would be less. One drawback to these sets was the Tyco style DC power pack that was included that started off with 6 volts to the track, so you could not run these engines at a crawl or even slowly.
My own layout has the option of running on either AC or DC current. I like those low cost DC only locos. They are easily improved visually and by adding internal weights to the engine (space that would have otherwise been taken by a circuit board), these engines will run both slowly and pull a nice train of 10 cars easily.
