The Hong Kong MTR commences service with the opening of its Modified Initial System (aka. Kwun Tong Line).

The Kwun Tong line is a heavy-rail rapid transit line of the MTR network in Hong Kong, coloured green on the MTR map. Starting at Whampoa in Hung Hom and ending at Tiu Keng Leng in Tseung Kwan O, Sai Kung, the route has 17 stations and takes 35 minutes to complete. The Kwun Tong line is one of the busiest railway lines on the network connecting the central and the eastern portions of Kowloon via Wong Tai Sin. The line is mostly underground, but includes a lengthy elevated section, and runs generally in an east-west direction. During the morning rush hour, the Kwun Tong line utilises 33 trains running at 2.1-minute-intervals to achieve a route capacity of 85,000 pphpd.Opened on 1 October 1979 as the first urban railway line in Hong Kong and the first operated by the Mass Transit Railway Corporation (MTRC), the Kwun Tong line operates over much of the original section of the "Modified Initial System", from Shek Kip Mei to Kwun Tong station, which it is named after. The line has seen the most changes in alignment of all the MTR lines, the most recent in 2016. It has crossed Victoria Harbour to serve Hong Kong Island using two separate routes in its history, though other lines have since taken over the harbour crossings; as such, the current route lies entirely within Kowloon except for Tiu Keng Leng in the New Territories. It has interchanges with four other lines: the Tsuen Wan line at Yau Ma Tei- Prince Edward station, the East Rail line at Kowloon Tong station, the Tuen Ma line at Ho Man Tin and Diamond Hill station, and the Tseung Kwan O line at Yau Tong and Tiu Keng Leng station.

The Mass Transit Railway (MTR) is a major public transport network serving Hong Kong. Operated by the MTR Corporation Limited (MTRCL), it consists of heavy rail, light rail, and feeder bus service centred on a 10-line rapid transit network serving the urbanised areas of Hong Kong Island, Kowloon, and the New Territories. The system included 230.9 km (143.5 mi) of rail in 2018 with 165 stations, including 97 heavy rail stations and 68 light rail stops.Under the government's rail-led transport policy, the MTR system is a common mode of public transport in Hong Kong, with over five million trips made in an average weekday. It consistently achieves a 99.9 per cent on-time rate on its train journeys. As of 2018, the MTR has a 49.3 per cent market share of the franchised public transport market, making it the most popular transport option in Hong Kong. The integration of the Octopus smart card fare-payment technology into the MTR system in September 1997 has further enhanced the ease of commuting on the MTR.

Construction of the MTR was prompted by a study, released in 1967, commissioned by the Government of British Hong Kong to find solutions to the increasing road congestion problem caused by the territory's fast-growing economy. Construction started soon after the release of the study, and the first line opened in 1979. The MTR was immediately popular with residents of Hong Kong; as a result, subsequent lines have been built to cover more territory. There are continual debates regarding how and where to expand the MTR network.The MTR has served as a model for some newly built systems, particularly in Mainland China.