The Air Force wants to upgrade its aging nuclear missiles and the hundreds of underground silos that hold them. One idea it's exploring: the construction of a sprawling network of underground subway tunnels to shuttle the missiles around like a mobile doomsday train. As one does.

As first reported by Inside Defense, the Air Force Nuclear Weapons Center will award several study contracts next month worth up to $3 million each to research the idea. A broad agency announcement from the Air Force describes the hair-raising concept, intended to keep the weapons secure through 2075, as a system of tunnels where nuclear missiles are shuttled around on rails or some undefined "trackless" system.

The advantage of the world's deadliest subway: During an atomic holocaust, mobile missiles are harder for an adversary to target than a static silo. Missiles could be positioned at launch holes placed at "regular intervals" along the length of the tunnels.

"The tunnel concept mode operates similar to a subway system but with only a single transporter/launcher and missile dedicated to a given tunnel," stated 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."

The Air Force hasn't given specifics on where the tunnels could be built, or how long they'd need to be. But they'll probably have to be jumbo-sized to "minimize impact from attack during all phases of missions/operations," the notice stated. The Air Force requires that all research proposals address ways to "minimize likelihood" that unauthorized persons could sneak in, while keeping the system working safely and not sacrificing the doomsday train's ability to "conduct world-wide operations."

The project would likely be gigantic, expensive and take decades to build – all things that cut against cut against these relatively lean times at the Pentagon. But the U.S.' silo-launched nuclear arsenal of 420 Minuteman III ballistic missiles are some of the oldest weapons still in service with the military, and they're only getting older. (Not to mention the upkeep the military has to perform on the other two legs of the nuclear triad, submarine-launched Trident II missiles and the air-dropped B61 nuclear weapon.)

The Air Force has spent billions upgrading the Minuteman's guidance systems, rocket motors and power systems to keep them serviceable through 2030. In a March 5 posture statement (.pdf) to the House Armed Services Committee, U.S. Strategic Command chief General Robert Kehler said the Minuteman IIIs are "sustainable through 2030 and potentially beyond with additional modernization investment." But to sustain the missiles until 2075, the service has to come up with new ideas.

The subway of doom isn't the Air Force's only option for revamping its silos. Others include "super-hardened" silos, or ground-based "transporter erector launchers" – really large trucks that can haul nuclear missiles around the country, including on public roads and even off-road. The trucks have their downsides: who knows how well they can cross bridges; people would freak out if they encountered nuclear missiles on their morning commute; and they're way more expensive than silos.

Last year, nuclear analyst and Danger Room pal Jeffrey Lewis estimated that building a fleet of 500 such mobile launchers would cost about $52 billion. "Apparently, building a 200,000 pound truck with rad-hard electronics and capable of withstanding nuclear blast effects is expensive," he blogged. Lewis also noted that the mobile nuclear launchers were too expensive during the Cold War when, y'know, full-scale nuclear war was a big threat.

Hans Kristensen, a nuclear analyst at the Federation of American Scientists, thinks the Air Force is stuck with plain old static silos. "The nuclear subway ICBM is, I think, a pie in the sky and more included to have a review process entertain a range of options so it can land on the most sensible," Kristensen tells Danger Room. "The costs associated with developing and operating such a system would be enormous and completely out of sync with the fiscal realities of this nation. Even a mobile system is probably unrealistic. I think the most likely, and probably only realistic option short of scrapping the land-based leg of the deterrent, is to simply extend the life of the existing Minuteman III ICBM."

Then again, maybe the Air Force defies logic and builds the death tunnels that it says it wants. Tomorrow's Armageddon could ride to work on rails.