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Starting with the London Underground in 1863, why are so many metros/subways built underground (as opposed to en elevated metro)? This requires a lot of digging and excavation, which to me seems a lot more expensive than an elevated metro like the Chicago L.

Paris, New York, Moscow, and others have also chosen underground metros. So there seems to be some almost universal reason of history. In fact the only major metro that's above ground, that I'm aware of, is the Chicago L.

Why was this? Was there no alternative? To me it seems an elevated system would cost no more than a bunch of bridges brought together, whereas big tunnels underground must be hugely expensive.

Was it something besides costs, such as the streets simply being too narrow for at-grade or above-grade system? Was there no good bridge-engineering in the early 20th century? Or is digging those tunnels somehow cheaper than an elevated system?

Edit: Because there is so much info here, I opened a related question about Why Chicago chose an L.

Rodrigo de Azevedo
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DrZ214
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    NYC does have elevated sections in Queens and Brooklyn with a short elevated section in Manhattan. – SMS von der Tann Oct 09 '16 at 00:40
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    Berlin has a mixture of underground, above-ground and and almost ground-level. – Paŭlo Ebermann Oct 09 '16 at 12:09
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    I suspect that this is really a question for [engineering.se], rather than here. Of course, it is a question about history, but essentially all the answers are engineering answers. – David Richerby Oct 09 '16 at 13:00
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    I think this rightfully belongs here at History as the NYC and London have different historical reasons, despite the engineering reasons. – Astor Florida Oct 09 '16 at 13:30
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    One concern, at least in NY, is that elevated trains were messy. Oil, soot, and grime was always dropping from the tracks to the road, so that some people would open umbrellas just to cross under them. – rougon Oct 09 '16 at 13:59
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    I thought it was just long term planning for movie chase scenes. I might be wrong, it happens occasionally. – corsiKa Oct 09 '16 at 14:41
  • @rougon Presumably they would use electric for either one. If they were digging tunnels, then probably they already had confidence in an electric system or else carbon monoxide and trapped pollution would make an underground system impossible. – DrZ214 Oct 09 '16 at 21:17
  • @DavidRicherby Yes I considered Engineering.SE, but the question there would have to be "Which is more expensive: 10 km of elevated metro or 10 km of underground metro?". (and really, I'd hafta add in the early 20th century too, because the situation might be different with technology today.) On history, the question is more why did so many choose one not the other? Price of construction is not the only factor – DrZ214 Oct 09 '16 at 21:25
  • The el cars were not regular coal-burning trains in the early days. Any electric upgrades would have been later. – rougon Oct 09 '16 at 23:22
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    You know Paris has quite a bit of above-ground metro too – Thomas Oct 10 '16 at 05:15
  • The red and blue lines are underground in Chicago when they go thru downtown. They are pretty shallow and I suspect they were trenched in. – Be Kind To New Users Oct 11 '16 at 14:42
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    Can we agree underground is better, right? Tracks can be more direct, go under buildings, parks, rivers, etc. So a better question would be how were the city planners able to convince people to choose the costlier, but the better long term option. – James Lawruk Oct 11 '16 at 17:27
  • @MichaelPotter there was an article yesterday about how the Red Line got under the Chicago River (and beyond a bit in either direction). Some was "cut and covered" (trenched as you suggest), but a large portion was also done with a tunnel boring machine, and some by "traditional mining". – Nick T Oct 11 '16 at 19:21
  • A modern elevated system is the Bangkok Skytrain – paj28 Oct 11 '16 at 22:58
  • The CTA has 224.1 miles of track, 11.4 of which are underground. – Mazura Oct 12 '16 at 19:44
  • You might be interested in reading the debate on Stuttgart 21, especially the "pro" side of it. – rumtscho Oct 15 '16 at 13:03
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    I image it is cheaper to dig through city owned underground than having to buy lots of buildings to knock down to make a path for an above ground system. Metros tend to be built in areas already heavily developed. – John Apr 13 '17 at 14:13

9 Answers9

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In Moscow, under former mayor Yuriy Luzhkov, it was built a line (Butovo line) which is mostly elevated. I think the practice was not considered quite successful as a result.

There are many drawbacks:

  • The elevated line still consumes ground area. Even though one can lay highways and streets under it, one has to demolish buildings. Not much different from a ground-level railroad actually. That said, the Butovo line was built at the city's outskirts, where the land was cheaper and had less buildings (especially historically important ones).

  • Greater levels of sound pollution. The extensive sound shields installed on the line were still unable to completely isolate the surrounding area from noise. Residential buildings have to keep a similar distance from this line as from any railroad.

  • The need for special roll stock. It seems that the carrige park started to wear off much quicker than at underground lines and required special climate protection. The same roll stock is impractical to use on the both elevated and underground lines, which adds logistic troubles. In summer the trains on the elevated line need conditioners, while at winter they require a heater.

  • Higher maintenance costs. The tracks need better care due to seasons. This includes cleaning of snow and ice of both rails and platforms and increased rail expansion due to different temperature in summer. Also, increased corrosion due to rain.

  • Higher construction costs. Overall, the cost of construction turned out to be $34M/km compared to $30M/km for cut-and-cover underground lines in Moscow and $22M/km in Kiev. This difference includes the need for specialized roll stock though.

That said, there is also a system of electric commuter trains in Moscow, so-called "elektrichka". They are mostly ground-level, share the track with intercity trains and the lines may go far into Moscow region beyond city limits. But inside the city they can be used the same way as metro, and often are cheaper.

Rodrigo de Azevedo
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Anixx
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    Even though one can lay highways and streets under it, one has to demolish buildings. Why is that? The elevated metro would simply follow an existing road (if it's wide enough), so no buildings would have to be demolished. Overall, the cost of construction turned out to be $34M/km compared to $30M/km for non-deep underground lines in Moscow and $22M/km in Kiev. Do you have a citation for that? I really want to read more into those costs and how they were calculated. – DrZ214 Oct 08 '16 at 23:42
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    @DrZ214 "The elevated metro would simply follow an existing road" - in that case the road should be made thinner. The same waste of area as for ground-based rail. Also, ground roads often to not follow optimal path because of land obstacles. Also, metro lines are often used to connect areas which are otherwise not connected directly by roads. – Anixx Oct 08 '16 at 23:44
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    @DrZ214 "Do you have a citation for that?" - sure, here it is: http://www.nashemetro.ru/line12.shtml It also compares that with Madrid where an underground line Metrosur costed $30M/km with all infrastructure (depots, rail stock) included. – Anixx Oct 08 '16 at 23:46
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    @DrZ214 notice that the underground lines can be built two ways: deep underground and non-deep underground. Deep underground lines are literally drilled in the earth with special equipment and quite expensive. Non-deep underground lines are first excavated with standard excavators as a trench and then covered with concrete ceiling. Non-deep underground lines are actually easier to build than elevated lines, but they are also subject to similar limitations (should follow an existing road for instance). But have no noise and do not take area except you cannot build solid buildings over them. – Anixx Oct 08 '16 at 23:56
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    Elevated lines (and non-deep probably too) also make for pretty lousy bomb shelters, which was a big deal during construction of older parts of Moscow subway. – Daerdemandt Oct 09 '16 at 07:46
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    There are also mixed systems. In Paris, line 2 and 6 both have elevated sections where they follow wide boulevards. They go underground at complex intersections or when they stop following roads. Also, though lines were first constructed in trenches, later ones are now all drilled. I would assume the cost of stopping the traffic in a busy city for months is a huge cost in itself; and drilling allows faster lines (not having to follow sharp turns). – spectras Oct 09 '16 at 13:55
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    @spectras in Moscow, the situation is mostly opposite. Under Stalin the most of lines were drilled, except the very first line. Then under Khrushchev, Brezhnev, Gorbachev, the lines were mostly trenched, with some ground-level ranges. Under Luzhkov the construction was mixed, there are drilled, trenched and elevated ranges. But overall, the majority of drilled lines are from Stalin's epoch. Most lines in the center are drilled. – Anixx Oct 10 '16 at 01:37
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    @Anixx, That is not exactly so. Since late 1960s, in the ex-USSR, the cut-and-cover method is mostly used for stations only. The tunnels between are almost always (even if shallow) built by tunnel-boring machines. Moreover, the metro has expanded greatly after Stalin, and deep sections built after Stalin outnumber the whole metro built under Stalin. – ach Oct 10 '16 at 05:14
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    @Andrey Chernyakhovskiy no, you are wrong, at least for Moscow. – Anixx Oct 10 '16 at 05:25
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    I should add that in densely built cities, elevated and sub-surface lines have to follow the street plan, which may place very harsh requirements on their technical parameters. Look at Chicago; even though the city has a regular grid of streets, junctions and turns are extremely tight (90 ft minimum radius of curvature) and necessitate the use of very short carriages. – ach Oct 10 '16 at 05:25
  • @Anixx, In which point? – ach Oct 10 '16 at 05:34
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    @Anixx. The standard term for what you're calling "non-deep" is "cut-and-cover": you dig a trench, then put a roof on it. – TRiG Oct 10 '16 at 11:57
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    @DrZ214: In city centres, existing roads tend to make tight 90 degree turns - something which is non-optimal for most rail cars. In cities that don't have too many skyscrapers packet close together like Kuala Lumpur it is easier to build elevated tracks. – slebetman Oct 10 '16 at 14:17
  • @TRiG I thought "deep" and "non-deep" simply meant drilling through bedrock and drilling (cutting) through soil respectively. The last one is not necessarily synonymous with cut-and-cover. You could be just installing a large sewer pipe, for example. – DrZ214 Oct 10 '16 at 22:58
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    @DrZ214, Rocky terrain is found in few xUSSR cities with metros. When talking of ex-Soviet metro building industry, 'deep' and 'shallow' are terms describing the building methods. 'Deep' structures tend to have round arch shapes to withstand the pressure of earth and are built using mountain mining techniques or a tunneling shield. 'Shallow' structures are flatter and are built with a variation of cut-and-cover technique. However, some 'deep' structures may be actually shallower than 'shallow' ones. – ach Oct 11 '16 at 06:02
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    @DrZ214 in most cities the road system is not a grid, they often wouldn't form lines that would be suitable for trains so simply following existing roads isn't even an option. – JamesRyan Oct 12 '16 at 11:20
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This is mostly about urban planning, and how much change the local government can or will be able to make to the existing streets.

In London, the central parts of the city (Westminster and the City) still have their street plan from medieval days, as the 1666 fire didn't burn the foundations. The streets there are far too narrow, and the buildings too important or politically unchangeable. When industrialization came in the 19th century, the trains couldn't go all the way into town. If you look at today's mainline rail maps, all of the stations are at the outskirts of the City.

The only remaining way to get trains in and out of central London was to put them underground. That said, once you get out of central London, the London Underground trains run overground, and occasionally on elevated tracks. Once they were out into the suburbs, it was possible to get planning permission for tracks and stations above ground, and that's what happened because it is much cheaper.

The same is true for Paris, Moscow and New-York. In all of these places, the city existed long before the trains did. Because of that, the trains were built in afterwards underground.

Since Chicago grew into a city after (or at least during) the industrial revolution, the city was planned and built to include the trains, or at least space for them to pass through where no historical structures stand. Because the trains weren't added to the city, but planned into it, they had the option of building them above ground.

Carmi
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  • Are there any elevated sections of the London Underground? Sure, there are various embankments and so on, but the ones I'm aware of are just to smooth out the terrain, as with any other railway line. I wouldn't describe those as "elevated". Also, when you talk about Chicago, you don't explain why planning the railways at the same time as the city results in railways above the ground, rather than just on the ground. – David Richerby Oct 09 '16 at 13:06
  • I don't think there are any elevated parts of the Underground, no. But there are plenty of other elevated railways in London; for example, some of the Overground in north London, and much of the commuter railways that take the place of the Tube in south and south-east London. See for example https://en.wikipedia.org/wiki/London_Bridge_%E2%80%93_Greenwich_Railway_Viaduct. – Daniel Roseman Oct 09 '16 at 13:31
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    Parts of the Docklands Light Railway near Canary Wharf are elevated. I'm not sure about the other lines. – CSM Oct 09 '16 at 15:11
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    There multiple elevated sections of London Underground (Hammersmith & City line in Sherpherd's Bush, District line in Putney, Northern line north of Golders Green, parts of Piccadilly line north of Turnpike Lane), but none are in central London. – Alexander Frost Oct 09 '16 at 15:47
  • It's true that London, Paris, Moscow, etc., have railway stations at the edges of the city centre, and underground metro lines. What I have often wondered is why none of them connected the long-distance railway lines underground, and upgraded one of those stations to a true central train station. – gerrit Oct 09 '16 at 20:46
  • I might ask a related question about why Chicago chose an L when so many others chose underground. – DrZ214 Oct 09 '16 at 21:15
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    @gerrit - several reasons: ownership, politics, money, geography. It's worthy of a whole question in itself. –  Oct 09 '16 at 21:38
  • Its worth noting that the elevated section of the northern line north of Golders green is at least partially influenced by the terrain. There is a relatively steep incline between golder green and hampstead, such that the deapest point below ground level, and highest point above ground level are both in that area. – user1937198 Oct 09 '16 at 22:38
  • @CSM The DLR isn't the Underground: it's the DLR. – David Richerby Oct 10 '16 at 07:17
  • @DavidRicherby The Central Line is mostly elevated as it heads toward Epping. – SGR Oct 10 '16 at 09:57
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    @gerrit the long-distance trains are of larger diameter, requiring much more expensive tunnels, and longer, requiring larger stations. Metro systems are generally smaller: London 3.56m, Glasgow's tiny 3.35m versus Eurotunnel's 7.6m. Having said that, there's now a large section of Eurostar track in tunnel on the approach to St Pancras. – pjc50 Oct 10 '16 at 10:52
  • @gerrit. The Thameslink line in London connects North-South across the city. Trains run through from Bedford to Brighton. – TRiG Oct 10 '16 at 12:00
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    In addition to @pjc's comment, mainline trains are less frequent and (in the centre of a major city) will be swapping many passengers in and out. Those passengers take up space while waiting, thus you'd either have (i) a large overground concourse (seriously large if all the main lines came to a single point) with the land requirements that entails; or (ii) a great cavern underground. That woudl be expensive to hollow out and support, and would push the trains even further down. – Chris H Oct 10 '16 at 12:45
  • @gerrit: Crossrail (or the Elizabeth Line as it's now known as) is a heavy-rail line with stations under central London. Opening in 2018, I believe. I don't know if you'd classify it as just a "commuter" train, or a "long-distance" train, but either way, they're pretty much the same thing. –  Oct 11 '16 at 00:49
  • It was intended at one point for mainline trains to run underground between Paddington and Liverpool Street, providing east-west through services along what is now the Circle/Metropolitan line. Underground trains and National Rail trains use the same gauge track. – Andrew Leach Oct 12 '16 at 10:37
  • @CSM The DLR goes over reclaimed marshland and docks so the reasons for it to be an elevated railway are through neccesity and not for the cost savings that the OP questions that applies to other open air parts of the underground (which tend to be towards the outskirts of london) – JamesRyan Oct 14 '16 at 09:48
  • @JamesRyan: I'm not sure that's true of the DLR, as the Jubilee line goes underground in the same area, so it's clearly possible and not prohibitively expensive. Mostly, the DLR runs in an area that was abandoned (more or less) for about 50 years, which is like building a new city, rather than attaching to an old one. – Carmi Oct 26 '16 at 06:26
  • @Carmi they are close to each other but if you look at them on a geographical map you will see why what you are saying is not true. The abandoned area you talk about building on is historic dockland, the DLR stations are a lot smaller and closer together than normal tube stations, plus the jubilee line is much newer and came after some of this area was reclaimed, together with linking high value locations that made tunneling more viable. – JamesRyan Oct 26 '16 at 10:48
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For New York, the answer is related to real estate value.

In New York City, the construction of the metro was performed by real estate developers. The idea was to build homes, then connect them to the city with a subway. Sales of the new homes, in principle, then funded the metro system.

Above ground trains were not conducive to high priced luxury apartments.

Example

Prior to the subway, Queens Boulevard had some small communities an a bunch of farms. The construction of the IND Queens Boulevard line coincided with a bunch of new suburban communities, with high rise apartments at express stops like Forest Hills and Kew Gardens, and smaller developments at other stops. Wikipedia mentions a property value increasing from $1,200 in 1925 to $10,000 in 1930; I think while this figure shows investments earning 10x in just a few years, this was just the tip of the iceberg. There were 21 new stops on this subway, each with more than a few mile of intensive real estate development.

More Info

While Wikipedia lists concepts like weather and right-of-way costs, real estate money was the key to development in New York because the subways were not publicly funded. This can be evidenced in another Wikipedia paragraph:

In Kings County, elevated railroads were also built by several companies, over Lexington, Myrtle, Third and Fifth Avenues, Fulton Street and Broadway. These also later shared trackage with subway trains, and even operated into the subway, as part of the BRT and BMT. Most of these structures have been dismantled, but some remain in original form, mostly rebuilt and upgraded.

I've always found it astounding when cities in the US actively destroyed parts of their transportation infrastructure. Typically, at least in New York City, the motives for many things related to real estate prices. Evidently, the increase in value of properties along these routes would increase if the elevated tracks were removed, so TPTB decided that they should be removed. Some were replaced with subways, some were replaced with buses, and some with private cars.

As mentioned in the comments, there are still some elevate lines, for example the number 7 train in queens, and the N train in Astoria, and the number 2 train in the Bronx. These areas were historically cheaper than the more expensive, and prestigious areas of Manhattan. Thus most of Manhattan put the subways underground to further increase property values. If things haven't changed to much, the only elevate line in Manhattan is around Washington Heights, where property values are cheaper than the Upper West Side or elsewhere.

What about Chicago?

Chicago is a great city - go Cubs! But it is not in the same league as New York, London, Moscow, Paris, and Tokyo. The urban core is much smaller, there are a lot less people, perhaps 1/4 or 1/3 the population of New York City.

What is interesting about Chicago is that they retained their elevated system. Los Angeles, Detroit, St. Louis, and a bunch of other cities had elevated systems and streetcar systems before the 1950s. These systems were all town down by General Motors, and replaced by bus lines. How did the Chicago "L" system survive?

Perhaps Chicago was big enough to need the system?

I am not 100% certain, but it appears that because of financial trouble in 1932, Chicago Edison Electric Utility divested control of the "L" prior to the Public Utility Holding Company Act of 1935. The city between 1932 and 1935 must have started to begin to gain control of the system, and eventually developed the CTA which took control in 1940, insulating the system from destruction by GM.

It is likely a combination of Chicago being larger than St. Louis and L.A. at that time, and the growth the CTA that saved the "L" from destruction in the 1940's. The reason most of it remain above ground likely related to it being historically much less expensive than Manhattan. I'll see if I can find a source to corroborate this thought.

Astor Florida
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Noise, vibrations, and visual impact drop the value of the nearest houses, and decrease quality of life for its inhabitants. The maintenance of an elevated system is expensive, not only economically but also in terms of streets closed to traffic for performance of maintenance work.

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Vicente J. Bailén Jonker
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    I have no idea why this was downvoted. Everything said is true. +1. – DrZ214 Oct 10 '16 at 09:15
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    I wasn't the down voter but this answer doesn't add anything that is not already mentioned in other answers and lacks any real world examples to back up its assertions. – Steve Bird Oct 10 '16 at 09:58
  • Cut-and-cover lines probably bring even more vibration than elevated lines. Only the drilled lines evidently have less vibration. – Anixx Oct 12 '16 at 21:47
  • Indeed, all one needs to do is go to the end of NYC's elevated NQ line at mid-block in Astoria, and compare the grimy feel of the half of the block in its shadow to the more open and charming feel of the half beyond the end. The noise of cut-and-cover lines is barely noticeable in an urban backdrop, but that of of elevated ones substantial. – Chris Stratton Oct 15 '16 at 19:03
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Quite often subway lines are built to relieve traffic pressure on areas where there is no room to add more roads, including the towers needed to allow for the creation of elevated roads or railways.

In fact, if you look at cities with subways, almost all of them have the tracks come above ground and continue as a sort of light railway at the point where the suburbs were under construction by the time they started constructing the subway system, precisely because it's so much less expensive to build above ground, and when they can do that while planning a suburb around the new metro/rail line they will.

London does it like that, as do Amsterdam, Munich, and many others.

TRiG
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jwenting
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  • Do you mean foot traffic or vehicular traffic? – rougon Oct 09 '16 at 15:40
  • @rougon both, but mostly vehicle traffic as that's the main problem (not just congesting roads but causing pollution and taking up valuable space when parking). – jwenting Oct 09 '16 at 16:08
  • Of course it's less expensive to build at exact ground level. I was comparing underground with elevated system (like Chicago L) where the track is like a continuous bridge running over the road. – DrZ214 Oct 09 '16 at 21:23
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    @DrZ214 those systems are much more dangerous in case of emergencies (harder to reach for emergency services), harder to maintain (and more expensive), and in earthquake zones extremely prone to collapse. – jwenting Oct 10 '16 at 11:07
  • @jwenting Personally I would not feel that safe in an underground tunnel during an Earthquake either, but I guess it would be easier to reach than an elevated system. – DrZ214 Oct 10 '16 at 22:54
  • @DrZ214 tunnels are easier to make sturdy enough to withstand an earthquake than are raised tracks, but I understand your apprehension :) – jwenting Oct 11 '16 at 02:58
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I can add that Washington DC is still expanding its Metro and the expansion is not underground. The newest line is the Silver Line, and as it goes through Tysons Corner it's almost entirely elevated. There was a loud grassroots movement to put it underground through Tysons Corner, but underground is much more expensive and risky.

The arguments against elevated lines echo what has been said in other answers: unattractive, extra maintenance and problems due to weather, and so on. An additional argument is that it's built along the middle of a couple of major roads that already make the area pedestrian unfriendly, and so it will always be an anchor against the ultimate goal of remaking Tysons Corner more of a mixed-use, pedestrian-friendly place like nearby Arlington.

Speaking of which, Arlington County fought long and hard (back in the 1970's) to force the Metro underground, instead of the cheaper alternative of running it down the center of I-66 (the nearby Interstate). They succeeded in arguing that it needed to go through Arlington's downtown rather than skirt and divide town, and the major changes to Arlington over the last 40 years bear that out. Arlington is a shining success story for an underground Metro that could have been above ground. It went from a sleepy, semi-industrial suburbs to a place that other cities point to as their goal. (Including Tysons Corner, but also from around the world.)

I'd suggest looking at this history of the Washington DC metro -- particularly Arlington and Tysons Corner -- for some nice historical lessons. (As a side note, one of the ritziest neighborhoods of Washington is Georgetown, and back in the day, Georgetown residents fought off the Metro stop that was planned for their neighborhood. Arlington County jumped at the opportunity and in the end, the money originally earmarked for Georgetown's Metro stop was used to build an extra Arlington Metro stop -- Virginia Square. Georgetown regrets this loss more with the passing of each year. Long-range plans call for more Metro lines to be added, and Georgetown is in line to get one when new lines are added -- someday. EDIT: This account has been disputed, and I've answered in comments. The Georgetown part may be partly urban legend, though I heard it from one of the people involved (on the Arlington side) at a lecture.)

EDIT: To add a bit more history, Arlington came up with a plan where the densest construction and highest buildings would be along the (underground) Metro line, falling off a bit between stops, and falling off quickly as you move away from the Metro line entirely and towards single-family-home neighborhoods. It hasn't totally followed that plan, but it's worked very well, and part of that is because you can build right on top of an underground Metro. So it's not always the case that you need underground because of pre-existing buildings that can't be touched: it allows a lot more room to build new tall buildings along your route.

My inner child would love to see an elevated monorail, ala Disney or Seattle's short monorail run, but...

Wayne
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  • FTR, Georgetown never had a serious metro stop planned for it - it's too close to the tunnel under the Potomac, and doesn't have enough offices in it. See https://www.quora.com/Why-is-there-no-metro-station-in-Georgetown-in-Washington-DC – JasonB Oct 12 '16 at 22:16
  • @JasonB: Plausible. On the other hand, for example, the intersection of M Street and 31st St NW is just as far from the Potomac as Rosslyn is, and Rosslyn is very deep because of the river, and back in the 1970's there were lots of Metro stops that didn't have many offices or any high-rises nearby. I heard the story not as an urban legend but from one of the Arlington folks who got Metro through Arlington underground -- they spoke at the premier of a video (I bought a DVD copy) at some anniversary of Metro or Arlington a few years back. – Wayne Oct 13 '16 at 00:04
  • I'd also add that if you look at an actual map -- not the stylized Metro map -- Virginia Square is visibly closer to its two neighbors (Ballston and Clarendon) than other stops on the Virginia Orange Line, so it's plausible that it was added late in the process. (Perhaps from somewhere other than Georgetown -- articles that I've read that disagree with Georgetown ever being seriously considered note that other locals fought against Metro stops, just as Georgetown did.) – Wayne Oct 13 '16 at 01:07
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In Berlin, Germany, there is - as said before - a mixture of underground and overground services. The reason here for not using underground in some areas is the consistency of the soil / kind of ground. So its an engineering reason.

Rathony
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Tarek
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There is a city Chongqing in central China, there are two big rivers & lots of rocky mountains so for some districts it was too difficult/too expensive to build underground metro (or "undermountain" metro) so some line are almost elevated.

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Ivan Gerasimenko
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The Metro in Moscow is underground (and is especially deep) because it was a dual purpose structure that would have been used as a shelter in case of a nuclear war. At least that's what I heard when I lived in Soviet Union. The decorations there were also very special and intricate to impress normal people with the Soviet future.

MCW
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nulleight
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    Factually incorrect. The metro in Moscow has a mix of deep tube, subsurface, overground and elevated sections. All underground stations were retrofitted in 1950s, and all new underground stations thereafter have been fitted to be able to serve as WMD shelters; but this is done opportunistically. The only period in the history when metro in Moscow was built deep underground specially for that purpose was late 1940s to early 1950s (the Stalin's paranoia period). – ach Oct 14 '16 at 18:21