Gyrocar

I came here to post the same thing!

The fuel economy on this was said to be excellent (because it's a tandem seating design), with the downside that the passenger was always relegated to the back seat...

Gyrocars also featured in the awesome science fiction novel Sinister Barrier, by Eric Frank Russell (published in 1943!).

Well, it _is_ wikipedia. You could add it.

There's a nice model of a gyro monorail shown in the video here: https://www.youtube.com/watch?v=jGYXJjRfgTM

I may be possible to do the same with Lego Mindstorms - certainly you can make self-balancing two-wheel robots: http://robotsquare.com/2014/07/01/tutorial-ev3-self-balancin...

Things that come to mind: less rolling resistance with 2 wheels, faster turning since you can lean, simpler mechanics in some aspects (no need for a differential), easier to build narrow vehicles with better aerodynamics.

Why have four wheels at the bottom, when you can simply have two on the bottom and two in the centre? :)

Why not just use transitor-based gyroscopes and lever the vehicle one way or the other when turning to keep the riders level? In other words, why not just treat the carriage portion of the vehicle like a large gimbal?

There is also this: http://litmotors.com/

But then you need to build the rocket to have reinforced attachment points for the rail sled and be strong enough to withstand the lateral forces during the rail launch, with the attendant cost in increased weight.