San Diego Model Railroad Museum in downtown San Diego's Balboa Park fills 27,000 square feet of the Casa de Balboa basement with the largest operating model railroad collection in North America, a distinction the only accredited model railroad museum in the country has held since opening in 1982. Four local nonprofit clubs — the San Diego Model Railroad Club, La Mesa Model Railroad Club, San Diego Society of N-Scale, and San Diego 3-Railers — build and operate the permanent layouts, a volunteer-driven model that mirrors the citizen-science tradition at San Diego Natural History Museum upstairs on El Prado. Five permanent layouts replicate real California rail lines with geographic precision: the 4,500-square-foot San Diego & Arizona Eastern traces the route from Union Station through Carrizo Gorge to El Centro, while the two-level Tehachapi Pass layout recreates the iconic loop curve-for-curve from thousands of prototype photographs. A Toy Train Gallery features a 42-by-44-foot three-rail O-gauge layout with locomotive-mounted cameras streaming a first-person perspective to gallery monitors, and the 10,000-volume Erwin Welsch Research Library preserves railroading photographs and blueprints on the same floor that Museum of Photographic Arts occupies upstairs in Casa de Balboa. School field trips, scout-group workshops, and summer camps use the operating layouts to teach engineering, geography, and California transportation history through hands-on train operation.