When the Minuteman ICBM program was conceived and developed (1959-1963), of the first 150 missiles, it was planned to deploy 1/3 of these weapons on specially constructed railroad cars on trains based out of Hill AFB, Utah running on the commercial rail network throughout the Western and Central United States.  The rest of the first 150 would be deployed in the standardized, prefabricated, reinforced concrete underground silo missile sites we know today.

The rail mobile system was tested and proven under Operation Big Star running 4 trial trains between 20 Jan 1960 and 27 Aug 1960 all over the operational area on simulated deployments looking for problems with the system and procedures and to get telemetry on the missile???s systems.  On 27 Aug 1960 the last test train returned to HillAFB, gathering all of the information needed in 4 missions instead of the 6 missions that were planned.  The system was declared successful upon arrival back at the base. 

SAC wanted the system, but the Air Force and the Kennedy Administration placed priority upon the silo basing option, deferring the mobile portion of the program on 28 March 1961.  In December of that same year, Secretary of Defense McNamara cancelled the mobile Minuteman program, along with the Skybolt air-launched ballistic missile (range 1150 miles, 1 Mt warhead, 4 rounds per B-52H on the inner wing pylons (some sources state up to 8 Skybolts with 4 carried internally)) and the DynaSoar X-20A program, among other things.

Causes for the cancellation of this part of the Minuteman program include costs, both set-up and lifecycle, security and politics/public relations.  It can only be imagined how much more of a problem this would have been ten and twenty years later even if the routes of the missile trains had been restricted to BLM lands.  It has to be remembered that at the time the silo basing was hardened more than enough to deal with the prevailing OPFOR threats.  Today that may well be less of the case considering the CEP ratings on some current OPFOR systems.  This was in part the causation for the Midgetman prototype and the original MX/Peacekeeper mobile basing proposals, especially the hardened off-road TEL vehicles.

Now getting back to air-launched options, the Skybolt, once all of the kinks were worked out, would have given both the USAF and the RAF a really flexible offensive option.  It would have put a lot more strain upon USSR air defense and ABM systems.  I have also seen proposals involving air launching Minutemen missiles in a satellite launch vehicle role from modified B-58s.  The Skybolt could also likely have worked in that role.  Heck, the Skybolt today would in may respects fit the Global Strike mission rather well.  Imagine the system with a modern guidance package and a non-nuclear payload option like EMP, bunker buster, smart submunitions, or FAE/thermobatics.