The Utrecht-Amsterdam railway opened on 6 December 1843. Utrecht became an important railway hub in the following thirty years as new lines were laid to Arnhem, Zwolle, Den Bosch and Hilversum. The railways were of great economic importance: the railway companies alone provided employment for large numbers of workers. But this ‘iron ring’ also hindered urban expansion for many years, and the coal fumes from the many trains sometimes hung over the city like a black cloud.