Although it's not part of the public tram tour, the Prop Warehouse is one of the stops on the extended VIP Experience tour.
A prop (or "property") is anything an actor touches during a film, from a paper clip to a grand piano.
Every item in the warehouse is separately identified, barcoded and catalogued. People searching for props for a production can easily find what they want from among the hundreds of thousands of items available. Spread over many floors of the massive Edith Head building, Universal's prop collection is the largest in the industry, containing over 1 million items.
Thanks to Phillip Donnelly, nfb_418, Josh Gross and Chris Barr for the photos.