Some prototype cars were built for launching the Minuteman III and MX missiles (and one or two of them are on display at the USAF Museum at Wright-Patterson AFB), but no complete trains were built and placed into service. The SALT II talks were the main reason for that, IIRC, but cost and running those trains on the rail network (which obviously is not state-controlled like the Soviet rail system was) were also reasons for dropping the idea.