Have you ever seen a railroad bridge shaped like a trapezoid? These bridges, called truss bridges, were invented in the 1840s. Even when bridge manufacturers switched from wood to iron, they kept building truss bridges. They're still very popular today.
Truss bridges are an efficient and reliable way to span short distances. They use a system of triangular supports to spread forces across the whole bridge. This gives them the ability to bear heavy loads, such as trains filled with freight, without breaking. Truss bridges are also very rigid. They don't bend or twist in high winds. They last for years with very little maintenance, and they can be built off-site and transported to wherever they're needed. There are quite a few varieties of truss bridges, but most were designed in the United States during the 1840s and 1850s. Companies competed to build the safest, strongest bridges. Historians classify the bridges based on how the triangular supports fit together. While the bridges were originally made of wood or iron, most truss bridges today are built from steel.
On some truss bridges, vehicles travel across the top of the trapezoid. On others, they travel across the bottom. Some designs, like the one pictured above, use alternating trapezoids to allow truss bridges to span greater-than-normal distances.
See for yourself: http://science.howstuffworks.com/engineering/civil/bridge4.htm
Visit the links below to see more trapezoidal truss bridges, learn about the different varieties, and find out what caused the unfortunate collapse of a truss bridge in Washington.