Complex sentence: There are nine fully underground stations on the network (six in Newcastle upon Tyne, one in Gateshead and two in Sunderland), and a further four sub-surface stations (Regent Centre, Chichester, Heworth and Byker).
Split into simpler sentences: There are nine fully underground stations on the network (Six in Newcastle upon Tyne, one in Gateshead and two in Sunderland). And a further four sub-surface stations (Regent Centre, Chichester, Heworth and Byker).