The need for reliable transportation systems evolved with the industrial revolution. By the early 19th century, it was necessary to transport materials, finished goods, and people over greater distances in shorter times. A large demand for railway bridges was generated as railroads in the UK and the United States prospered and expanded. European and American engineers realized that a more ductile material was required to resist the tensile forces developed by heavy railroad locomotive loads. The first railway bridge in the United States constructed entirely of iron was a Howe truss with cast iron compression and wrought iron tension members built by the Philadelphia and Reading Railroad in 1845 at Manayunk, PA. Suspension bridges were more economical but many British engineers were hesitant to use flexible suspension bridges for long-span railroad crossings. The basic forms of ordinary steel railway superstructures have not changed substantially since the turn of the 20th century.