The Newport or Hillfield tunnels are two parallel bores, of 678.5 metres (2,226ft) length. The earlier one is Brunel's 1850 South Wales Railway with monumental rock-facings and oversize portals (as Twerton, St.Annes and Box on the Great Western Railway). The second bore is a twin-track standard-gauge, the north tunnel of 1912, with a matching gaping portal in engineering brick. The northern end of the tunnels is at National Grid Reference ST306879.
Source: A Guide to the Industrial Archaeology of South East Wales, AIA, 2003
Claire Parry, RCAHMW, 21 June 2011.