Most likely because a railroad probably won't run a train with a broken winsheild for more than one reason ( I'd imagine that this would break some FRA law). Not to mention fixing a winsheld isn't that hard, heck railroads have rebuilt entire pieces of an engine such as the cab. (Look up NS 8352 and notice the nose of the cab.) Honestly though even if they break something in a movie that goes to something that will be used after the movie it's usually fixed.