A short line is an inependent railroad company that operates over a relatively short distance. Short lines generally exist for one of three reasons: to link two industries requiring rail freight together (for example, a gypsum mine and a wall board factory, or a coal mine and a power plant); to interchange revenue traffic with other, usually larger, railroads; or to operate a tourist passenger train service. Often, short lines exist for all three of these reasons. Because of their small size and generally low revenues, the great majority of short line railroads in United States are classified by the American Association of Railroads as Class III. In Canada, Transport Canada classifies short line railroads as Class II.