There are three grades of railroad crossings over streams. The first is a culvert where a metal pipe is run perpendicular to the tracks to carry storm water. In this case bedding and gravel is used to support the rail. The second type of crossing is a trestle.
This is a treated wooden structure used to support the railroad tracks. The tracks are supported by large timbers and cross braces to support the loads. Timber is used in place of ballast. The third type of crossing over a stream is a reinforced concrete bridge or a steel bridge.
This type of bridge is used to span 90 feet or more and used for major crossings where streams are impacted by the 100-year storm. Again this type of crossing may or may not use gravel to support the ties and railroad track. Source: Budget responsible manager Department of the Army subcontractor. For more information contact the American Association of Railroads standards.