Complex sentence: The underpass is built within part of the former Kingsway tramway subway, which was 20 ft (6.6 m) wide and allowed bi-directional flow because of the fixed rails and relatively narrow width of the trams.
Split into simpler sentences: The underpass is built within part of the former Kingsway tramway subway. The original tramway tunnel was 20 ft (6.6 m) wide and allowed bi-directional flow because of the fixed rails and relatively narrow width of the trams.