Complex sentence: Parts of the city underground were constructed and exist as the present-day City Circle, with small sections built for the additional proposed city lines such as additional platforms at Wynyard and St James railway stations which have never been used for heavy rail transport.
Split into simpler sentences: Parts of the city underground were constructed and exist as the present-day City Circle. Small sections were also built for additional proposed city lines, such as additional platforms at Wynyard and St James railway stations which have never been used for heavy rail transport.