Complex sentence: A third track was added between Hawthorn and Burnley in 1972, which allows the centre track to be used as a bi-directional track for express services during peak hour and completed triplication of the line as far as Box Hill.
Split into simpler sentences: A third track was added between Hawthorn and Burnley in 1972. This allowes the centre track to be used as a bi-directional track for express services during peak hour and completed triplication of the line as far as Box Hill.