Back in 85-89 the company I worked for had the monumental task of complete restoration of The Union Station washington DC /Amtrack station also. I was part of a team of constuction professionals and During the four years I worked /lived there I had 100% access surface & substructures .
some of these Sub structures were tunnels some built back in the mid 1800's .I explored tunnels that were litterally 300 + feet under steet level. Some were proven to go to the white house and the capital. These tunnels, most of which had RR tracks in them, some were put there secretely as our historical engineers could not find any archival documetation that they existed . These Sub structures were problematic to us developing this site and all had to be IDed within 500 ish feet of UnionStation.
We did find a 1860's train.. locomotive and a few cars some what sealed off in a tunnel.. historical people said it was lincolns "escape train"
I found that ALOT" thousands" of "street People"=tunnel people live and breed, die down there Generations of um. Rats that look to eat you were everywhere .
There has been a few different documentaries though the years reporting tunnels from DC to the mountains of WV.(Nemicolin if I remember right )
So how deep is deep enough to hide ... how far is far enough to go ...when everybody want to harm you And your children ?
I'm sure every PRES with a guilty consious as dug a bigger deeper hole