A spring used as the mass of a flywheel to improve weight / energy storage ratio.
A spring is good for acceleration from- and breaking towards standstill, a flywheel is better for more linear deceleration and acceleration. When the spring is unwound it continues to serve as a flywheel. It's more or less left to spin free. Breaking force is still transformed onto the spring, force applied to the spring gradually turns into the rotation of it-self. In the same way the flywheel is slowed down in order to accelerate the vehicle or to generate electricity.
The large weight can be of further help to keep a land sailing vehicle on the ground. The sailtrain would have 2 rows under each compartment those at the left spinning in opposite direction from those at the right to avoid gyroscopical effects.
Because the diameter of the spring decreases the flywheel accelerates while the spring is set. While the spring is released the speed decreases and the force goes up, this can be used to some-what smoothen out the decrease in force the spring applies.