The Mario Kart Go! set is officially licensed by Nintendo, and features about 20 feet of track (we're not slot car racers, so we don't know if that's good or bad). The set is apparently popular, because it's already sold out, with a new shipment due in this Tuesday, December 12th.
We'd prefer to see Yoshi, Luigi, or even Bowser as Mario's slot car rival, rather than Wario, but who can complain? Mario Kart and slot car racing is truly a match made in heaven.