To cut construction costs, the authority used an abandoned canal bed to build part of the system. Thus the underground section is only 2–3 meters beneath the street surface, and was the world's shallowest metro. Years later, train frequency reduced and trains were commonly delayed. The rolling stock had become dilapidated, with most seat covers torn off, and lights becoming dim. To change to a more modern and cleaner system, a modernization plan was laid out in 2000. In preparation, the system was closed on 1 September 2001, with renovation starting on 21 November after being delayed due to the terrorist attacks of 11 September. After introducing new rolling stock, adding half-height platform screen doors and extending the line to , the line was completed at the end of 2005 and re-opened on 12 June 2006.