If there's one thing Popular Science has taught me about retrofuturism, it's that even the most improbable ideas never die. They crop up, go dormant for twenty years, and reappear once a new generation of visionaries grows tired of the status quo. Take, for example, the super speedy, aero transit monorail. In 1919, a French inventor proposed combining an airship, a fuselage, and a train to create a propeller-driven monorail with wings. We were interested, but skeptical that it could actually reach 150 miles per hour without using wheels. Eleven years later, a Scottish inventor built a similar train, but with an overhanging rail instead of wings. We were excited for awhile, but got distracted by magnetic levitation. In 1972, long after Japan unveiled its Shinkansen trains, we began to suspect that it'd be decades before we could implement that technology. So we began dreaming of hanging trains again, but this time, minus the propeller.