Bill Higgins wrote:

original scheme where-- to keep spy satellites guessing-- the MX would ride
around on railcars concealed somewhere in a miles-long underground rail
network.

If I understand correctly, on launch the roof of the tunnel was to be blown
off, then the missile was erected and launched.

Later, this scheme was abandoned in favor of more conventional silos.

IIRC, the miles-long underground tunnel was an early concept that did
not progress very far. Before the eventual conventional silos, the
basing methods that was further developed were the multiple protective
shelters or "racetrack", and the "dense pack".

Racetrack had 23 (IIRC) horizontal shelters spaced some distance apart
on a closed loop road. Each racetrack would have one MX on a launcher
that would be moved between the shelters. The shelter spacing was
intended to be far enough apart to require an RV (or two) targeted to
each shelter for a counterforce attack against the MX. The shelters
were an evolutionary descendant of the buried coffins used by Atlas.

Dense pack put super-hardened silos in a small area of land so that
RV's targeted on nearby silos would face fratricide from each other.