Way back when, my daughter did a behind the scenes tour at Disney World (I don't think they offered such tours to adults back then). During that tour, they visited the facility where they store out of service monorails.

Also, when we stayed at the Contemporary - probably more than 10 years ago, we would often sit on our balcony (Magic Kingdom view) and watch them take the monorails out of service in the evening. It appeared, they would change to another track and head into storage for the night.