Also, if the barge itself is ionized along the underbelly, (and only the tube's *floor* is similarly ionized, for repulsion) then the tube ceiling could be veined with conductive rails, whose current-rise pulses are timed to the passage of the barge beneath them. The current-pulse generates a local magnetic field; the barge, being a fast-moving tub of ions, feels a reaction force, no? Or have I muddled my solenoids? At least it's magnetic thrust, as you suggested. :)