My most recent nerd trip to New York was to tour the New York City Transit Authority’s 240th Street Yard, also known as Van Cortlandt Yard. The train yard is completely elevated, which in my opinion makes for impressive structural engineering. It is a small shop, so it cannot hold all the trains that are out of service. The do regular maintenance as well as repair. I was impressed with all the safety mechanisms and protocols they have to make sure no one gets hurt.