Gravity shunts roll train cars downhill to save electricity

At the Neasden Depot in London, the world's most sophisticated rail network relies on the simplest force in the universe: a gentle slope. When engineers need to park hundreds of carriages across 93 miles of track, they use gravity shunts to let uncoupled cars roll into position unassisted. By allowing these massive vehicles to glide downhill into their designated berths, the depot saves 500 kilowatt-hours of electricity per move—roughly the same amount of energy a typical household uses in six weeks.