A gravity car is a train car that's towed up to a hilltop or some elevated area by a locomotive and then allowed to coast back down on its own. Gravity cars were not uncommon in the 19th century, and used in Marin County on Mount Tamalpais to take tourists to Muir Woods.
The train tracks that the gravity cars glided down are long gone, but the trails the tracks were laid on form a network all over Mount Tam in Marin, where I grew up. I started mountain biking on Mount Tam, and found out about "Gravity Car Grade". It's unremarkable for biking, being a gentle grade and broad enough to, well, run a train through. But it struck me as a great online handle, because it still sounds high-tech (if you don't know what it is) and is a little mysterious. I've hung onto it through the years, and when the time came for me to buy a URL, amazingly enough gravitycar.com hadn't been taken.