Most of the Hong Kong MTR runs underground in tunnels, but there are sections of track open to the outside world. Here is a list.
Island Line
Entirely in tunnel, except for:
- viaduct Heng Fa Chuen to Chai Wan
- Chai Wan depot at grade
Tsuen Wan Line
Entirely in tunnel, except for:
- viaduct Lai King to Kwai Hing
- Tsuen Wan station and depot at grade
Kwun Tong Line
Entirely in tunnel, except for:
- viaduct Kowloon Bay to Lam Tin
- Kowloon Bay depot at grade
- short cutting between Lam Tin and Eastern Harbour Tunnel
Tseung Kwan O Line
Entirely in tunnel, except for:
- LOHAS Park station at grade
- Tseung Kwan O depot at grade
South Island Line
Entirely in tunnel, except for:
- viaduct between Nam Fung Tunnel and Ap Lei Chau
Tung Chung Line and Airport Express
Entire line above ground, except for:
- tunnel between Hong Kong and Kowloon stations
- short tunnel at Lai King station
- tunnel through Tsing Yi
- tunnel at eastern end of Lantau Island
- underground station at Tung Chung
Disneyland Resort Line
Entire line at grade except for short tunnel south of Sunny Bay station.
West Rail Line
At grade after exiting the Tai Lam Tunnel in the New Territories, continues on viaduct to terminus at Tuen Mun.
East Rail Line
Entire line at grade, on embankments or in cuttings, except for:
- Tunnel No. 1A in Hung Hom
- Beacon Hill Tunnel between Kowloon and the New Territories
- Tunnels No. 5 and 5A at Tai Po Kau
- Long twin tunnel between Sheung Shui and Lok Ma Chau
Ma On Shan Line
Entire line is either viaduct or at grade.
Further reading
The MTR之今昔 website has a full track diagram of the MTR network, including detail of where tracks are in tunnels.
Hey do you happen to know the decisions behind making at grade, above ground or underground? I assume at grade is cheapest but most obstructive, viaduct if possible (when at grade is impossible) and if not, underground is final (but most expensive) option.
You’ve pretty much captured the cost reasons, with another consideration being visual impacts on the surrounding area, and availability of land.
Some more thoughts on the matter are here:
https://history.stackexchange.com/questions/33255/why-are-so-many-metros-underground-isnt-that-more-expensive-than-an-elevated-s
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