Impractical? How? A constant ‘lock on’ of every train in motion along an already mapped path. Signal sent from satellite to weapons system with updated, real time coordinates. The Russians might be able to ‘get a few off’ but after that all ‘stealth trains’ AND ANY OTHER TRAINS would be obliterated.
Not impractical operational wise. We have the technology to design and make such system operational. But keep in mind Russia is a very large country with 6 time zones, which is twice as many as United States. There are hundreds, if not thousands of trains running at any one time, mostly with all civilian passengers. Russia has the longest rail tracks in the world.
To blow up all passenger trains along with one nuclear warheads carrying train will kill more civilians than were killed in Hiroshima and Nagasaki. It will be the worst public relations disaster. That is why it is 100% impractical. Russia has more deliverable nuclear warheads than United States. Can you even imagine the retaliation?