[jjk166]

The Us heavily looked into mobile land based missile launchers but they really don't make sense. For transportation that uses infrastructure (road based, rail based), most of that infrastructure is built close to population centers and is critical - you don't want to make it more of a target than it already is and security would be a nightmare. You can build lots of infrastructure in the middle of nowhere but that's super expensive. For off-road vehicles you have to develop such a mobile launcher with all its technical difficulties, you limit the size of your missiles, and you need to set aside much larger areas of land than a missile silo field - basically you're just making a worse version of naval deterrent. The most economical option is silo fields with just extra silos that you periodically transfer warheads between, but the cost of transferring the warheads back and forth regularly is more than just having a fully functional nuke in every silo, and dummy silos make arms control verification nearly impossible.

Perhaps some of the assumptions underlying these rationales have changed in the intervening decades, but I don't see any of these options getting more practical than they used to be.