Clive Coleman. wrote:

I used to have a picture a little underground train that carried two
people and operated between the white house and some main station in
Washington D.C. This must be at least 30 years ago and was meant to
be secret. Years before Washington got a metro proper. If it can
happen there, it can happen here. I'm not asserting that is has,
but I wouldn't be surprised.

I don't see that a railway to be used only in emergency makes economic
sense. Surely a tunnel with no running rails would require less maintenance.