The tunnels were enlarged by removing several of the cast iron segments from each tunnel ring, excavating a void behind to the required new diameter and reinstalling the segments with additional packing spacers. The northern section of the C&SLR between Euston and Moorgate was closed from 8 August 1922, but the rest of the line remained open with enlargement works taking place at night. A collapse on 27 November 1923 caused when a train hit temporary shoring on the incomplete excavations near Elephant & Castle station filled the tunnel with soil. The line was briefly operated in two parts, but was completely closed on 28 November 1923.