Complex sentence: Initial service was confined to the Main North Line to Gosford, until the Glenbrook tunnel was widened in 1975, and to Lithgow after the Lithgow tunnels were widened in 1979.
Split into simpler sentences: Initial service was confined to the Main North Line until they were usable on the Blue Mountains Line after the Glenbrook tunnel was widened in 1975. They could proceed as far as Lithgow after the Lithgow tunnels were widened in 1979.