RYNO Motors' single-wheeled electric scooter is self-balancing, has a turning radius of zero when stopped, & only takes up about as much space as a folding bicycle -- so it's conceivable that commuters could bring one onto a train.
OneWheel Self-Balancing Electric Skateboard Feels Like A HoverBoard! Has a top speed of 12 mph, supposedly so intuitive that anyone can learn to use it within a matter of minutes! Shut up and take my money
Marbel is raising funds for Marbel - The Lightest Electric Skateboard in the World on Kickstarter! The World's Lightest Electric Skateboard. Just 9 lbs, up to 20 MPH, mile range and a mobile App to control & customize your board