This gets its purpose regarding plants a bit wrong, the only time roots would be any issue is on tracks which have not been used in quite awhile and at that poi

This gets its purpose regarding plants a bit wrong, the only time roots would be any issue is on tracks which have not been used in quite awhile and at that point the tracks will almost certainly have other issues which need to be addressed before they get used again. The metal on metal of the wheels on tracks often causes sparks and these do occasionally cause fires in dried vegetation alongside/on the tracks which the ballast helps to keep in control, which is why the ballast generally extends further from the edges of the tracks than is needed for stability reasons. If you work track maintenance part of your job will be torching any plants growing too close to the tracks especially on grades where wheels often slip and brakes applied which can produce a surprising amount of sparks. They also have cars which spray weed killers alongside the tracks which run the line once or twice a year depending on conditions.