You are correct about decoders for Lionel and other 3 rail O. The 3 rail O folks use TMCC (made by Lionel and licensed to others) for command control and/or DCS (made exclusively by and for MTH). The electronics in the locomotives is made to use 16 volt AC, and so is not compatible with the DC motor feed coming from a DCC decoder.
You could run Lionel on DC, but would have to bypass the electronics and associated sounds. The motors themselves all will work just fine on DC. Depending on the actual motor used, the polarity reversal of direction may not be available without additional wiring inside the locomotive (some motors have wound fields instead of permanent magnets).
If I were going to run Lionel on DC (DCC), I would buy the non-command locomotives and start from there. Bypass the reversing unit and sounds, and install an O scale (rated for a couple of amps) DCC decoder. But it's not trivial if you are not familiar with electronics.
hope this helps