It may very well be that the canopies are to keep rain out of the escalators, but the roll down doors are at the entrances at ground level. This keeps people from going down to the bottom of the stairs and taking a dump, or the top for that matter. The first of these was built in downtown Oakland and proved the concept. They are being installed in various locations as time and money become available. I would think body excreta would cause more damage to escalators than just rain.