Good luck finding an inverter that will fit inside with all the other electronics. I tried for a while but gave up. FWIW... The S scale Legacy electronics will operate on DC power, but I don't think it will work as TMCC and Legacy rely on the ground in your home to your track to complete the signal path. At least that's how I understand it. Choosing a "Dead Rail" system and starting from scratch would probably be your best bet.
I hope Lionel eventually creates a battery capable system with the quality of RailSounds.