Rick, one of the items not mentioned is how the signal gets to the engine to operate in command control. With DCC or DCS the signal goes through the track, through the wheels and into the decoder. So if you have clean wheels you will have a clean signal, dirty wheels dirty signal.
With TMCC/Legacy the signal goes through the rail but instead of progressing through the wheels it goes into what we call the tunnel effect on the rail. The signal forms like a tunnel around the track and then picked up from an antenna and then into the electronics. It doesn't care if you have clean or dirty track you will always get a clean signal.
Either system has its good and bad points but as a side note, you can buy DCC decoders from several manufactures and TMCC components are available from Lionel and ElectricRR Company.
Hope this helps.