http://insidedefense.com/20130...ans/menu-id-926.html
... the Air Force Nuclear Weapons Center last year outlined five potential alternatives to explore in an analysis of alternatives. These options, according to the January notice, include the continued use of the current Minuteman III baseline missile until 2075, with no attempt to close identified gaps; modest modernization approach that incorporates incremental changes to the current Minuteman III baseline to close the gaps; "new fixed" missiles; "new mobile" ICBMs; and a "new tunnel" option.
Of these approaches, the most bold -- and potentially most expensive -- would be the "new tunnel" concept. It would require a vast underground subway-like network of pathways to shuttle new missiles around to multiple launch portals, any of which of could be used to fire the missile. "The tunnel concept mode operates similar to a subway system but with only a single transporter/launcher and missile dedicated to a given tunnel," states the notice. "The tunnel is long enough to improve survivability but leaving enough room to permit adequate 'rattle space' in the event of an enemy attack."
Unmanned cars, either on rails or in "trackless" mode, would move along the tunnels and, in a doomsday scenario, use any one of the launch portals that would be built at "regular" intervals to allow the transporter to raised and fire the missile.