This isn't just freezing. Dry ice is far colder than wet ice. Whether the ambient temperature is above/below 0c won't make that much difference. The lasers w

This isn't just freezing. Dry ice is far colder than wet ice. Whether the ambient temperature is above/below 0c won't make that much difference. The lasers were never meant to heat the rails. Heating a shiny metal is very difficult. The leaf-cleaning laser system works specifically because it doesn't heat the metal. It bounces off the metal surface, heating only the non-shiny gunk on the rail.