liquid nitrogen would require better insulation at the least, since its much colder. I suspect they're also taking advantage of the fact that dry ice is solid,

liquid nitrogen would require better insulation at the least, since its much colder. I suspect they're also taking advantage of the fact that dry ice is solid, so they're effectively sand-blasting the track (rather than merely pressure-washing it), but they're using sand that helpfully sublimates into the atmosphere without leaving residue (or poisoning anyone). I imagine blasting sand or chemicals has the downside of leaving residue that can build up, meaning regular cleanup is required, and care required to use enough but not too much, with this you can spray as much as you can afford to buy/carry on the train and you don't have to worry about over-doing it. Dry ice blasting is already a thing, so I guess they just realised it was a particularly promising technology for this application.