Complex sentence: The C&SLR was planning a new northern extension to Moorgate and was going to close the poorly-sited King William Street station and sections of its two running tunnels under the River Thames, replacing them with a new pair of tunnels on a better alignment.
Split into simpler sentences: The C&SLR was planning a new northern extension to Moorgate and was going to close the poorly positioned King William Street station and sections of its two running tunnels under the River Thames. These were to be replaced with a new pair of tunnels on a better alignment.