A 5800CF open-topped hopper can hold 200 metric tons of pea gravel (at least according to the people selling them) so you just doubled the J/$ of the Powerwall. This is also using new rail cars, if you buy used then the price for the same rail car drops down to the 20K range according to online rail car brokers. Now we are looking at 1/4 the price for a train car solution.