Photo of a man working at the Balboa Park Model Railroad Museum. At 28,000 square feet, the museum is the world's largest operating model railroad museum. Model railroading in Balboa Park began at the 1935 California Pacific International Exposition. In the 1930s, pioneer model railroader Minton Cronkhite designed and directed the construction of a number of scale model railroads, including a large exposition O scale model railroad in Balboa Park. The museum opened in 1982.