Thanks, Mike.
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Cool.
Put the DC operated switcher on the other throttle, dialed in 0 and she ran (one digitrax can run two engines at once, two knobs). The switcher didn't run very fast but I'm guessing a CV can be changed for that. Kind of like the old Lionel: AC but put a DC 'override' onto the rails.
Another nice option for digitrax; don't know if the other systems have the ability to run one DC engine.