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For those who have the book, the explanation and example of a runaround is the first page of the first chapter of Track Planning for Realistic Operation -- page 6 in the Third Edition.
Right, unfortunately not practical for model railroads. Maybe with a surreptitious power truck in the car under separate DCC control. But that would be a lot of work for a parlor trick.
But if someone could build a setup that worked and was able to pull it off at a show or somewhere with a good size crowd watching, he/she would certainly make a hit!