In June 2000, MTRCL proceeded with plans to retrofit 2,960 pairs of platform screen doors at all 30 underground stations on the Kwun Tong, Tsuen Wan, and Island lines in a six-year programme. The programme made MTR the world's first railway to undertake the retrofitting of PSDs on a passenger-carrying system already in operation. A prototype design was first introduced at Choi Hung station in the 3rd quarter of 2001. The scheme was completed in October 2005, ahead of the forecast completion date in 2006. MTRCL said that part of the cost had to be assumed by passengers. HK$0.10 per passenger trip was levied on Octopus card users to help fund the HK$2 billion retrofit programme. This levy was ended in 2013 after raising more than HK$1 billion.