Box Tunnel is a railway tunnel in Western England, between Bath and Chippenham, dug through Box Hill, and is one of the most significant structures on the Great Western Main Line. It was originally built for the Great Western Railway under the direction of the GWR's engineer Isambard Kingdom Brunel.The tunnel is 1.83 miles (2.95 km) in length, straight, and descends on a 1 in 100 gradient from the east.