The railroad began in 1924 by the founder Mr. Hennan Howard and his brother who were members of the Pomona Model Yacht Club. A single Pacific Electric Railway 1200 interurban model ran around the Army Corps of Engineers' Puddingstone Reservoir Exhibit during that year's Los Angeles County Fair in Pomona. The train diorama was to promote the dam, which had just been built. The exhibit later was moved to a tent, and in 1935 to its outdoor location. The railroad was sold in 1958 to Mr. & Mrs. Herman Templin. After the death of Mr. Templin, just before the fair opened in 1968, Mrs. Templin, with the help of her daughter Suzanne and Mr. John Huie, kept the railroad running. In 1970, the Los Angeles County Fair took ownership of the railroad, and Mr. John Huie, under an agreement with fair management, kept the railroad maintained and running through 1996.
The exhibit ran every year for the Los Angeles County Fair except during World War II. The War Era brought a halt to the Fair for six years when the grounds were taken over by the U.S. Army. The first year the fair and the Outdoor Miniature Railroad resumed operations after WW II was in 1948.