Flywheel Batteries were an invention that allowed for more power to be used than a compulsator. This worked well in trains.
(Note: The background section of this page uses most of the same words as Terra Futura's page on compulsators to save time.)
Michael Faraday was a scientist was famous for his research into electricity and magnetism. He started out as secretary to Sir Humphry Davy of the Royal Institution in London. Sir Humphry Davy had been blinded in a chemical accident. After Sir Humphry Davy passed away, Michael Faraday experimented on electricity and magnetism. In 1831, Faraday discovered electromagnetic induction. This became the basis for the alternating current generator, or alternator. In 1891, Nikola Tesla invented the first practical alternator. With that, alternating current won the Current Wars. In the 21st century, these rotating alternators became compensated.
The compulsator was first invented in the 1970s at the University of Texas Center for Electromechanics for nuclear fusion research. An external power source spun the compulsator to allow for inertia to be stored. The compulsator was capable of storing energy from its rotors for short-term use. The rotors depended on high velocity spin. Too fast and the blades would break. Despite that, the compulsator was still being used in railguns. The large pulses of power that a compulsator produced were ideal for firing a railgun projectile that could really kill. The compulsator could only be used for short-term purposes. Flywheel energy storage could be used for long-term purposes.
Tech Level: 11
Flywheel batteries were based on the same concept as compulsators. Flywheel batteries spun in near-vacuum conditions. This allowed them to store more inertia than a compulsator. This made electric trains that did not use overhead wires or a third rail possible. Flywheel batteries were also used on the International Space Station for extra power. By the mid-21st century, flywheel batteries were being used for personal applications. Buildings became more autonomous. And yes, they were used in railguns and other weapons. Eventually, however, flywheel batteries were replaced with superconducting batteries.