Bob,
To kind of echo what Fred said, an engine with a certain decoder installed, ie Digitrax, will not only run on another DCC system, such as Atlas, MRC, Lenz, etc. but will also still operate on a DC layout. You won't have contol of the lights or sound, (if equipped) but it will still work. In fact, the only concearns when selecting a decoder is does it have enough amps to run the loco you want to put it in, is there enough room inside the loco and do you want any special lighting effects or sound.
2slim