A railroad bridge is a type of bridge that is specifically designed to carry a railway line over a body of water, valley, or other obstacle. The two most common types of railroad bridges are the TRESTLE and the VIADUCT. A TRESTLE is a bridge composed of a number of short spans supported by closely spaced frames or piers. A VIADUCT is a bridge composed of several small spans supported by tall piers or arches. Both trestles and viaducts are used to span large distances and can be constructed from a variety of materials, including steel, concrete, and timber.