Santa Susana is a former railroad town located mostly within the City of Simi Valley. A small portion of the community, outside the Simi Valley city limits to the south of the Ventura County Metrolink rail line, is an unincorporated area and census-designated place. The community is in the eastern part of the Simi Valley. The town by the Santa Susana Mountains in the Simi Valley was founded in 1903, shortly after the Southern Pacific Company built the Santa Susana Depot.