Spring steel energy storage? Heavy. Hard to move heavy rolling stock
with windpower alone. If you had an electreic booster motor running
off of a wire you could feed excess energy back to the rail
electrification system, hopefully not underfoot. Electric generator
brakes feeding an overhead or trackside wire might provide some power
for trains going dead upwind or uphill, but I doubt it could be
practicable. Better a cable drive, with counterbalanced opposing
direction cars, boostable with water ballast.

Rainwater is one way to bleed energy from wind, with a fog farm on a
hilltop.

One type of analysis could consider hydrogen as a battery electrolyte,
or cold liquid nitrogen as a sink for energy, permitting a cold engine
system, would utilise the atmosphere as rhetorical electrolyte storing
heat from liquifier pumps to be returned to the cold air hog "unsteam"
engine as energy to expand while heating the liquid and gaseous
nitrogen. Bonus: free air conditioning in the vehicle.

Timing the N2 condensation, or storage phase, and expansion, or use
phase, in combination with efficient insulation could increase the
apparrant energy density of the system if the N2 was stored after being
condensed in the cold of winter, warming the ambient, providing excess
heat for use in the winter, then if the N2 were "burned off" in the
summer, it could seem quite poweful and efficient, harvesting the
seasonal temperature variations.

It's a solar energy cycle thing, isn't it?

A sail snail rail train will never cut it. An air conditioned moped
with balloon weather shelter, a liquid N2 cold air hog might.

Why do we not use air conditioners as heaters in cool weather, saving
heating power, since a heat pump is the most efficient heater there is
(150%). All we need do is turn the air conditioner around in the spring
and fall, or not use it to cool in the summer, leaving it reversed all
the time, with only the plug in the wall as an off on switch.
Provision would also need be made to reroute condenser drippage.

Terry K