Complex sentence: The tunnel was built by the Great Western Railway (GWR) between 1873 and 1886, it is 4 miles 624 yd (7,008 m) long, although only 2¼ miles (3.62 km) of the tunnel are actually under the river.
Split into simpler sentences: The tunnel was built by the Great Western Railway (GWR) between 1873 and 1886. It is 4 miles 624 yd (7,008 m) long, although only 2¼ miles (3.62 km) of the tunnel are under the river.