[flashingleds]

Some time back I described a project to do this with a single data track (http://www.flashingleds.net/swipecards/swipecards.html)

This was a crude first pass, which is evident from the fact that the shim is a sawed off kitchen knife. In principle if you make the electromagnets smaller you could fit three. Make them adequately decoupled and you're away.

If these guys have an alternative approach I would be interested to read about it, but as far as I can tell they don't describe the method anywhere.