The problem with iron bridges was that the material was not as reliable as carbon steel, so it was replaced starting in the late 1800s and was completely unavailable for bridge building by the early 1900s. Bridges in the late 1700s were made of cast iron, but, by the early 1800s, cast iron gave way to wrought iron.
In 1857, the first wrought iron girder railway bridge was built in Germany and named the Weichsel Bridge. The oldest existing railway, however, can be seen in Australia. It was built in 1863 and can be visited today. It consists of two wrought iron riveted box girders and is called the Menangle Viaduct.
Changes began occurring in the mid 1900s when welding was discovered and made it possible to create better steel bridges. Since the 1930s, most of the large bridges that were being build were being built out of steel.
An example of these steel bridges is the George Washington Bridge, which was a suspension bridge built in 1931. Another example of these bridges would be the Golden Gate Bridge, which was built in 1937 above San Francisco Bay. Today, most bridges are built out of what is called composite construction, which uses both steel and concrete. Using the two materials together is the ideal for large spans and provides the greatest economic benefits.