Jeff,
I have a couple of older 2 rail MTH locomotives with PS 2 and they will also run on straight DC too. The sound, lights, and smoke function but you do not have the ability to operate the bell or whistle. Fun none-the-less. I always did find it amazing that MTH was able to come up with a system that would run on AC, DC, or DCS, and in the case of the PS 3 locomotives even DCC. Pretty versatile.
Mike