Gunpowder was also extensively used in railway construction. At first railways followed the contours of the land, or crossed low ground by means of bridges and viaducts, but later railways made extensive use of cuttings and tunnels. One 2400-ft stretch of the 5.4 mi Box Tunnel on the Great Western Railway line between London and Bristol consumed a ton of gunpowder per week for over two years. The 12.9 km long Mont Cenis Tunnel was completed in 13 years starting in 1857 but, even with black powder, progress was only 25 cm a day until the invention of pneumatic drills sped up the work.