A railway bridge is a structure that allows a railway line to cross over a road, river or other obstacle. The two most common types of railway 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 trusses. A VIADUCT is a bridge composed of several small spans supported by piers or abutments. Both trestles and viaducts are used to carry railway lines across obstacles such as roads, rivers and valleys.