Short answers, pulled from the story.
The LGM-118 Peacekeeper was an intercontinental ballistic missile designed to survive a Soviet first strike and then destroy hardened targets like enemy missile silos. It could carry up to ten W87 nuclear warheads, each with a yield of 300 kilotons, on a single rocket. The goal was to ensure that even a small number of surviving missiles could inflict massive retaliatory damage.
Only 50 Peacekeeper missiles entered service, despite plans to deploy as many as 200. Congress limited the deployment in July 1985 because no survivable basing solution had been agreed upon. Budgetary concerns and treaty obligations further constrained the program.
The last Peacekeeper was deactivated on the 19th of September 2005, during a ceremony at which the 400th Missile Squadron was inactivated. The START II treaty had made the missile's multi-warhead design a liability, since that treaty limited each ICBM to a single warhead.
The Peacekeeper used the Advanced Inertial Reference Sphere, or AIRS, developed from research at the Charles Stark Draper Laboratory. AIRS replaced traditional mechanical gimbals with a sphere floating in fluorocarbon fluid, achieving a drift rate of just 1.5 times 10 to the negative fifth degrees per hour. Platform drift accounted for no more than 1% of the warhead's final positional error.
After retirement, the W87 warheads were transferred to the Minuteman III missiles still in service. The Peacekeeper rocket stages were converted by Orbital Sciences Corporation into the Minotaur IV, a civilian satellite launch vehicle. Seven Minotaur IV flights had been made using old Peacekeeper components.
The Peacekeeper Rail Garrison was a plan to deploy 50 Peacekeeper missiles on the US rail network so they could disperse during a crisis and avoid destruction in a first strike. Each of twenty-five trains would carry two missiles. The plan was cancelled because of defense budget cuts following the end of the Cold War, and the missiles were instead installed in fixed silo launchers.