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Maidenhead Railway Bridge (aka Maidenhead Viaduct) is a railway bridge carrying the main line of the Great Western Railway over the River Thames between Maidenhead, Berkshire and Taplow, Buckinghamshire, England. It crosses the Thames on the reach between Bray Lock and Boulter's Lock
The railway is carried across the river on two brick arches, which at the time of building were the widest and flattest in the world. Each span is 128 feet, with a rise of only 24 feet. The flatness of the arches was necessary to avoid putting a "hump" in the bridge, which would have gone against Brunel's obsession with flat, gentle gradients (1 in 1,320 on this stretch).
The Thames towpath passes under the right-hand arch (facing upstream), which is also known as the Sounding Arch, because of its spectacular echo.
It has been claimed that the board of the Great Western Railway did not believe that the arches would stay up under the weight of the trains and ordered Brunel to leave the wooden formwork used to construct the arches in place. However, Brunel simply lowered the formwork slightly so that it had no structural effect, but appeared to be in place. Later, when the formwork was washed away in floods, but the bridge remained, the strength of the arches was accepted.
As built, Maidenhead Railway Bridge carried two lines of Brunel's broad gauge track. Subsequently the bridge has been widened, and now carries the four lines of standard gauge track that make up the Great Western Main Line from Paddington.