In June 2000, The MTR Corporation proceeded with its plans to retrofit 2,960 pairs of platform screen doors at all 30 underground stations on the Kwun Tong Line, Tsuen Wan Line, and Island Line 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. HK$0.10 per passenger trip was levied on Octopus card users to help fund the HK$2 billion retrofit programme. The whole installation scheme was completed in October 2005. The original completion year was 2006. The MTR Corporation said that part of the cost had to be assumed by passengers.