Initial section The Kwun Tong line operates over the majority of the track used by the "Modified Initial System", and can so be said to be the first MTR line to enter service. It was predated only by the suburban
East Rail line, which at the time was in the process of
being electrified and upgraded to a commuter service as part of the
Kowloon–Canton Railway. Construction was approved in November 1975 under the administration of Governor
Murray MacLehose, and service commenced on 1 October 1979. The line initially ran between
Shek Kip Mei station and
Kwun Tong station, and each train consisted of four cars. The trains were expanded to consist of 6 cars and 8 cars later respectively. It was extended to the south twice: firstly to
Tsim Sha Tsui on 31 December 1979, and secondly to
Central station on 12 February 1980 (named
Chater at the time), crossing
Victoria Harbour through the first underwater rail tunnel in Hong Kong and completing the original Modified Initial System plan. are set up at the three common stations of the Kwun Tong and
Tsuen Wan Lines. When the
Tsuen Wan line started service in May 1982, it took over the section of the Modified Initial System south of
Argyle (present-day
Mong Kok). At that point,
Waterloo (present-day
Yau Ma Tei) station became the terminus of the newly christened Kwun Tong line (until then, the line had no official name), and both Argyle and
Prince Edward stations became
cross-platform interchange stations with the new line.
Eastern extension When the
Hong Kong government decided to build a second harbour crossing in 1984, it awarded a franchise for the construction of
a mixed rail and road tunnel under the harbour. Consequentially, the Kwun Tong line was extended through the new tunnel on 6 August 1989 to a new terminus at
Quarry Bay, a transfer station with the newly built
Island line. An intermediate station,
Lam Tin, was opened on 1 October of the same year. The first
derailment in MTR history (excluding ex-KCR lines) took place at
Kowloon Bay station in 1994. The seventh carriage of a train pulling into the station at about jumped the tracks on 28 January 1994, on a section of track adjacent to the MTR headquarters building. Nobody was injured, though train services were disrupted. The incident was blamed on a
bolt in the train's
suspension system which had worked itself loose, causing the weight load to be concentrated on the rear wheels of the carriage. As part of a project to reduce congestion at Quarry Bay, the Kwun Tong line was briefly extended to
North Point on 27 September 2001. This station did not last as the terminus for long, as the newly built
Tseung Kwan O line would take over the cross-harbour portion of the route in 2002. On 18 August 2002, The Kwun Tong line was extended to
Tiu Keng Leng, its present eastern terminus, coinciding with the opening of the Tseung Kwan O line. Although not in regular service, the original tunnel linking the Kwun Tong line to the Eastern Harbour Crossing continues to be maintained and can be utilised in the event of a disruption on the Tseung Kwan O line. Such an incident occurred on 16 December 2013, when a train on the Tseung Kwan O line broke down, halting train services on the entire line for several hours. To prevent cross-harbour train service from being disrupted, all Kwun Tong line trains temporarily used the old tracks from Lam Tin to Quarry Bay and terminated at North Point, as they did before the opening of the Tseung Kwan O line.
Western extension As part of its bid for
Sha Tin to Central Link (SCL) in the early 2000s, the MTR Corporation proposed an extension of the Kwun Tong line to serve the
Whampoa Garden area, with an interchange at
Ho Man Tin to an extended
Ma On Shan line, which would provide an alternate route to the Tsuen Wan line across Victoria Harbour to
Central South station. Its competitor, the
Kowloon-Canton Railway Corporation, suggested constructing an
Automated People Mover between
Hung Hom and Whampoa instead. After the
MTR–KCR merger in 2007, the Hong Kong government appointed the MTRC to construct the SCL between Tai Wai and Hung Hom according to the KCRC's modified proposal, which would see the Ma On Shan and
West Rail lines merged to form the
Tuen Ma line, while also extending the Kwun Tong line to Whampoa as per the MTRC's own proposal. The benefits would be a better transfer arrangement at Ho Man Tin and other SCL interchange stations for services to the northeastern and northwestern New Territories. Passengers would be able to change to the
North-South corridor at Hung Hom for cross-harbour services, which would terminate at
Admiralty after Central South station was removed from the final plan. The
Kwun Tong line extension (abbreviated
KTE; ) from
Yau Ma Tei to
Whampoa via
Ho Man Tin began construction on 25 July 2011 and opened for service on 23 October 2016. Because of capacity limitations due to the single platform at Whampoa, half of all Whampoa-bound trains terminate at Ho Man Tin during peak hours; all trains terminate at Whampoa during off-peak hours. A further extension of the Kwun Tong line to Hong Kong Island was also proposed in RDS-2000 to constitute the fifth harbour crossing. ==Rolling stock==