Composition of scum on windows will change along length of tunnel.
Entrance end windows will be stained or etched by detergents. Most of the time replacement makes more sense.
Middle sections may be covered by grey or reclaim water. Use weak acidic solution like when cleaning soft-cloth brushes.
Exit end will be covered with drying agent residue, etc. Use a solvent to remove scum (i.e. mineral spirits).
After cleaning, coat windows with rain-x or similar to minimize scum.