Complex sentence: The first attempt to build the tunnel was abandoned, with the southern portal still visible, but the second attempt was successful, and on 29 November 1925, the North Auckland Line was complete and the isolated Northland railways were linked to the national network.
Split into simpler sentences: The first attempt to build the tunnel was abandoned, with the southern portal still visible. However the second attempt was successful, and on 29 November 1925, the North Auckland Line was complete and the isolated Northland railway lines were at last linked to the national network.