Self-Balancing Gyro Monorail Brennan
Long before bullet trains, there was the self-balancing gyro monorail from the early 1900s. Its inventor, Louis Brennan, filed his first monorail patent in 1903, and the initial prototype was just a 30.0″ by 11.8″ box containing the balancing system.



This prototype impressed the Army Council so much that they recommended a sum of £10,000 ($428,353 USD in 2023) for the development of a full-sized vehicle. Unfortunately, the funding was vetoed by their Financial Department, but the Army did end up finding £2,000 ($85,670 in 2023) from various sources to fund Brennan’s work. The basic idea was a vehicle running on a single conventional rail, which required a spinning wheel mounted in a gimbal frame whose axis of rotation (the precession axis) is perpendicular to the spin axis. The assembly is then secured to the vehicle chassis such that, at equilibrium, the spin axis, precession axis and vehicle roll axis are mutually perpendicular.

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Self-Balancing Gyro Monorail Brennan
One major downfall is that many cars, including the passenger and freight cars, not just the locomotive, would need a powered gyroscope to stay upright. Its gyroscopic balancing system is statically stable, so that the control system serves only to impart dynamic stability.
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