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I've heard and read mixed reports on this. At our club, running on 00 was banned (although I know that some still do it). They are afraid that although is is allowed by our Digitrax system, it has the potential for 1) harming the loco; 2) screwing up ops by shorting out the system; 3) harming the system.
While, I've heard of 1) and possibly experienced 2) (could have maybe been caused by a DCC loco in a similar circumstance), I doubt the system can be harmed.
At any rate, you don't want to leave a DC loco idling on a siding unless you want it to gasp that last white smoke life. Others will caution that extended running of DC may cause the same reaction.
Anyway, our club decided to ban it. I do it occationally at home to test a loco I'm working on before I convert to DCC.
On the flip side running DCC on DC depends on the decoder. Digitrax and Quantum switch back and forth automatically. The Lenz that comes with my Atlas silver has a jumper. The SoundTrax (at least the non-Tsunami) make white smoke.