From the ‘40 on, when there was a derailment on a major RR line, little or no effort was made to salvage freight, including automobiles. They couldn’t shut down a line for that long. There are dozens of sites where RR cars full of new automobiles were bulldozed into trenches and covered up. The insurance companies paid off and the cars are still buried in the middle of nowhere. Most were probably banged up pretty good but old RR hands told me that some were buried without a scratch. In arid areas, they probably still don’t have a lot of rust.
FDR had the CCC string them together to form sand dunes, flash flood channels, or something, out there.
Sort of balanced off the breakwaters he had built where it was wet.
As to this 'secret' junk yard; seems an enterprising state could contract with someone to collect and sell off the good stuff to at least partially cover cost of cleaning up the rusted out hulks.
(They are not all mustangs; some nice stuff in there; '46 Ford, IH Pickemup, some F100s, couple of Mopars, Torino and Gran Torino, some '49-52 Chevies, Econolines, at least one vintage trailer, etc. Even the 'stangs have good fenders, top bows and hydraulics, back seats ($500 for just the wires for a convertible) and other very hard to find parts - and all [yes, even Mustang] are better quality than any repop.)
And, someone wrote about where all those Mustangs went - NO BODY, in the entire world, Ford or Ford owner, thought ANY Mustang would last more than about ten years...they wren't built to go any farther.