According to the START treaties the TOPOL trucks can't just be located anywhere... there were specific buildings they needed to be housed in normally... and it would be different with trains because START is over.
This meant that at times of heightened tensions they knew where to start looking for some pretty distinctive trucks that would be fairly unique on Russian roads... only able to make certain turns so lots of roads they could not drive down anyway.
Compare that with trains that look the same as hundreds of thousands of other trains they already operate and it is not just a case of trains operating at higher speed and being given green lights where every they are... and if they make them high speed trains with speeds of 400km per hour or faster... perhaps a test track in the middle of the arctic where it is military land and anyone can be shot on sight with drones patrolling the area continuously... all the train has to do is stop and turn off on to a siding and raise its missiles and launch them which would take less than 5 minutes... then it does not matter if you blow them up.
More importantly the trains could be monitored... coordinating attacks on all the trains would need sophisticated communication and coordination... as soon as one is attacked the rest can be launched immediately anyway... For every attack there is a very good chance what they are attacking is a troop transport or cargo transport to a Russian military base or port, but those forces tasked with the attacks will quickly reveal who is attacking and where the response should be directed... a high speed train could be based in a mountain base but with a 400km/h speed they could have other tunnels all through the ural mountains where they can run to and launch from if there is any question as to who to attack.
Adding a carriage and another long range cruise missile would be cheaper than building another silo or SSBN...