Riverside is a city in Platte County, Missouri, United States. For many years the town was known for its Riverside Race Track. The Riverside Park Jockey Club operated and was popularly called Pendergast Track after its patron Big City Boss Tom Pendergast (although Pendergast was not officially on its organization papers). The horse racing track was on the site of a former dog racing track. The track operated under a questionable legal basis.