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So freezing the surface of the rail so the detritus freezes and disintegrates should do alright, but I wonder how the line or section of line could get taken out of action when the line is already below freezing. Wouldn't it become a layer of ice then, or is that when we get the laser out to heat the rails?

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It's more like sand blasting, but with dry ice instead of sand. This has been used for paint prep in industry and automotive for some time now. https://en.wikipedia.org/wiki/Dry-ice_blasting

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.

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