Securing track, if it needed to be done, wouldn’t be all that difficult. Not just trains can travel those tracks. Smaller vehicles can with an adapter called hyrail.
Nope. It is extremely difficult to secure railroad tracks. Thousands of miles of track have to be protected wide enough that a sniper outside the secure perimeter can’t put an incendiary bullet into a liquid oxygen (or for MX, solid) fuel tank of a road-mobile ICBM.
Thousands of miles of track have to be defended against remote IEDs, contact mines, magnetic shift mines, and plain old rail damage (which can come from nature via falling boulders as easily as from an enemy).
Rail-mobile ICBM’s (MX) were studied in depth. Both rail-versions of the MX missile got canned for solid reasons (the “racetrack” option and the “rail-garrison” variant).