The northern side of the Thames was the more populous, housing two-thirds of London's population, and the works had to proceed through congested streets and overcome such urban hurdles as canals, bridges and railway lines. Work began on the system on 31 January 1859, but the builders encountered numerous problems in construction, including a labourers' strike in 1859–60, hard frosts in winter, and heavier than normal rainfall. The rain was so heavy in June 1862 that an accident occurred at the works re-building the Fleet sewer. The deep excavations were running parallel to the excavation of a cutting at Clerkenwell for the Metropolitan Railway (now the Metropolitan line), and the wall dividing the two trenches collapsed, spilling the waters of the Fleet onto Victoria Street, damaging the gas and water mains.