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Switzerland authorizes removable PV plant on railway track
(www.pv-magazine.com)
This is a most excellent place for technology news and articles.
Maybe they only put them on the "Off Road" Tracks of the Street Trains. They usually have a covered bottom (like Cars) to avoid debris flinging into its soft parts.... but that will only solve one Issue of the many MANY there are. (Lots of Ifs here)