• Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!

How Do Landscape Types Affect Infrastructure?

Status
Not open for further replies.

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
This primarily applies to rapid transit systems, but the same principles I will bring up can apply to other railway networks, including those in the United Kingdom. But by landscape types, I mean such as forests, deserts, plains, mountains, flat green areas etc.

Firstly, let's do a bit of comparing. I've noticed that the rapid transit systems (aka. metros/subways) in cities like London and New York City, have a considerable amount of the infrastructure underground with all kinds of stations and tunnels, which sometimes run separately in the case of the latter. Yes, London Underground is slightly more above ground, but the point is that the rapid transit systems in these cities, which are located in green areas such as valleys or plains (I don't exactly which) have a considerable amount of underground infrastructure.

By contrast, the Dubai Metro is predominantly elevated above street level, with only a handful of stations even running underground, and often these are large single-bore tunnels that don't run on for anywhere near the distances. Unlike London and New York City, Dubai is built on very sandy desert landscape, and unlike rocks such as limestone, I can't imagine sand holding up very well against digging beneath the surface unless there's some sort of pre-fabricated tube placed ahead to minimise the settlement. But I'm acting on assumption.

This doesn't just apply to rapid transit, tunnels are built outside big cities too. But the same principles will apply. But in the case of national networks the case of electrification also comes up. I've never seen a railway with overhead lines in the middle of a desert (granted some of these barren landscapes won't have much hospitable places to reach out too), but they're the primary source of electrification for railways in countryside for example. Ultimately the question is whether the types of landscapes will affect the building of railway infrastructure in the case of tunnels, bridges and electrification, and if so why this may be the case.

I understand that somewhere like New York is a more densely packed and bustling metropolis than the less densely populated Dubai with more open areas, but are these the only reasons why New York Subway is primarily underground while Dubai Metro is primarily overground? Do populated and hospitable areas also effect whether a railway will reach these areas, hence why electrified railways may be less common in deserts than countrysides?

I may have worded it badly, but if you can just about understand what I'm getting it, do please give your best answer if you can. Thanks.
 
Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R

RailUK Forums

GRALISTAIR

Established Member
Joined
11 Apr 2012
Messages
7,807
Location
Dalton GA USA & Preston Lancs
Tunneling under sand is not easy. Tunneling under chalk marl or rock is much easier. Building skyscrapers on sand is not easy but very easy on rock. Conversely if you have a lot of skyscrapers you have to avoid all those when tunneling. Loads of factors. Thanks for starting this thread -should be interesting.
 

Old Yard Dog

Established Member
Joined
21 Aug 2011
Messages
1,466
There isn't a bridge between the Wirral and Liverpool as the underlying geology is too weak to take the weight.
 

DarloRich

Veteran Member
Joined
12 Oct 2010
Messages
29,182
Location
Fenny Stratford
I don't understand the question: What are you asking? I think i know what you mean but would like to be clear.
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
I don't understand the question: What are you asking? I think i know what you mean but would like to be clear.

Is it just me or do we have serious problems understanding each other? :p Perhaps you could tell me what you think I mean and maybe I could work from that and tell you what you might've got right or wrong with your understanding?
 

DarloRich

Veteran Member
Joined
12 Oct 2010
Messages
29,182
Location
Fenny Stratford
Is it just me or do we have serious problems understanding each other? :p Perhaps you could tell me what you think I mean and maybe I could work from that and tell you what you might've got right or wrong with your understanding?

I think you are asking: How do environmental factors affect the selection and installation of railway infrastructure.
 

GRALISTAIR

Established Member
Joined
11 Apr 2012
Messages
7,807
Location
Dalton GA USA & Preston Lancs
I think you are asking: How do environmental factors affect the selection and installation of railway infrastructure.

I do NOT think he is asking that. Sure some comments on this thread may well discuss environment. I think he is asking more about geography geology and possibly wider geopolitical factors. My opinion
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
I think you are asking: How do environmental factors affect the selection and installation of railway infrastructure.

Pretty close actually. I was more or less actually referring to the geology of the area such as why there are more tunnels in rapid transit systems in desert cities compared to cities in green areas, and also why there are less overhead line-powered railways in desert areas than there are in countryside areas. I want to know how the landscape around a railway affects the construction of one, and why sometimes a tunnel isn't possible just an example.
 
Last edited:

AndrewE

Established Member
Joined
9 Nov 2015
Messages
5,075
Pretty close actually. I was more or less actually referring to the geology of the area such as why there are more tunnels in rapid transit systems in desert cities compared to cities in green areas, and also why there are less overhead line-powered railways in desert areas than there are in countryside areas. I want to know how the landscape around a railway affects the construction of one, and why sometimes a tunnel isn't possible just an example.
...except that geology has very little to do with whether an area is desert or "green."
Cities are usually where they are because of historical trade routes etc (Liverpool, London, New York) or industrial factors like the availability of coal or water power and railway infrastructure largely depends on the money available at the time the need is recognised, tempered by the geology. Hence little Underground (metro) expansion in S London in the '30s, and almost none elsewhere. New tunnels in New York recently though, even though they have had to bore through hard rock. (http://www.nytimes.com/2012/08/05/magazine/tunneling-below-second-avenue.html)
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
...except that geology has very little to do with whether an area is desert or "green."
Cities are usually where they are because of historical trade routes etc (Liverpool, London, New York) or industrial factors like the availability of coal or water power and railway infrastructure largely depends on the money available at the time the need is recognised, tempered by the geology. Hence little Underground (metro) expansion in S London in the '30s, and almost none elsewhere. New tunnels in New York recently though, even though they have had to bore through hard rock. (http://www.nytimes.com/2012/08/05/magazine/tunneling-below-second-avenue.html)

Apologies. I'm no expert on geology. I should've just said landscape. But do you know if the landscape in Dubai affects the new metro? The majority of it is above ground elevated over street level, with a short section underground but in a single bore tunnel. Does the fact the city sits on sand have any effect on what tunnels can be built compared to boring through limestone or hard rock like in London where many tunnels can exist alongside, above and below each other?
 

Llanigraham

Established Member
Joined
23 Mar 2013
Messages
6,074
Location
Powys
Apologies. I'm no expert on geology. I should've just said landscape. But do you know if the landscape in Dubai affects the new metro? The majority of it is above ground elevated over street level, with a short section underground but in a single bore tunnel. Does the fact the city sits on sand have any effect on what tunnels can be built compared to boring through limestone or hard rock like in London where many tunnels can exist alongside, above and below each other?

But the landscape is affected by the geology.
As has been said, it is not easy to bore a tunnel in sand whereas it is much easier in rock. Sand collapses, rock, especially chalk, self supports.
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
But the landscape is affected by the geology.
As has been said, it is not easy to bore a tunnel in sand whereas it is much easier in rock. Sand collapses, rock, especially chalk, self supports.

Do you know how much would you theoretically be able to bore through sand? Would it be very limited like in Dubai, or could it be done on mass scale but would be more expensive because of all the self-supporting equipment if it could be done on mass scale? Another thing too, are desert rocks self-supporting as well? I did wonder whether desert rocks meant that it wouldn't be possible to do a subway in some areas of Texas (though I later discovered that the primary reason is because of the population density and whether it'd be viable compared to other forms of transport). Perhaps the underground Dubai Metro sections are in rock, but don't quote me on that.
 

Andyjs247

Member
Joined
1 Jan 2011
Messages
707
Location
North Oxfordshire
Cities are often where they are because of the geography and/or geology. Proximity to the sea for trade; a convenient crossing point of a river; a location where existing routes join or cross; somewhere near to mineral deposits; a strategic location that could easily be defended. Whether or not Railway Infrastructure is in tunnel or on the surface may be related to geology but I’d say it’s more likely dependent on price and availability of land. That is likely to depend on whether the city has been planned with roads and railways in mind or grew organically before cars and railways existed. It also depends on the economic value of the land. I’d say the overriding consideration for infrastructure is economic, though it is influenced to an extent by geology. Hope this helps.
 

dviner

Member
Joined
7 Oct 2010
Messages
246
Tunneling through sand may not be easy compared to the tunneling involved for Crossrail or the London Underground, but isn't it more a case of the tunneling methodology being used?

I'm not a civil engineer, but wouldn't driving a tunnel through sand be more along the lines of excavation rather than boring? Cut and Cover may even be a better option...
 

GRALISTAIR

Established Member
Joined
11 Apr 2012
Messages
7,807
Location
Dalton GA USA & Preston Lancs
Cut and cover maybe but while you are cutting you would get sand flowing back and while you are covering there would be no solid foundation on which to cover. Not easy. Chat Moss was not easy and that is not even sand. Yes I know that was not cut and cover
 
Last edited:

edwin_m

Veteran Member
Joined
21 Apr 2013
Messages
24,793
Location
Nottingham
Firstly, let's do a bit of comparing. I've noticed that the rapid transit systems (aka. metros/subways) in cities like London and New York City, have a considerable amount of the infrastructure underground with all kinds of stations and tunnels, which sometimes run separately in the case of the latter. Yes, London Underground is slightly more above ground, but the point is that the rapid transit systems in these cities, which are located in green areas such as valleys or plains (I don't exactly which) have a considerable amount of underground infrastructure.

By contrast, the Dubai Metro is predominantly elevated above street level, with only a handful of stations even running underground, and often these are large single-bore tunnels that don't run on for anywhere near the distances. Unlike London and New York City, Dubai is built on very sandy desert landscape, and unlike rocks such as limestone, I can't imagine sand holding up very well against digging beneath the surface unless there's some sort of pre-fabricated tube placed ahead to minimise the settlement. But I'm acting on assumption.
In pure construction cost, building at ground level is a lot cheaper than building a viaduct, which is a lot cheaper than building a tunnel. But other factors such as land prices and existing buildings or roads mean the cheapest option isn't always possible, either for economic reasons or because the local authorities won't allow it.

Much of the Dubai Metro runs along Sheik Zayed Road, the country's main highway. It was relatively easy to find space for the piers of a viaduct but finding a strip wide enough for a surface route would have been much more difficult. Also there are frequent side roads so the Metro would have had to go over or under each one. All in all far easier just to put it on viaduct. The tunnels are near the older part of the city, which I haven't visited but where it is probably more difficult to carve a path through for a viaduct. Other metros under construction or planning in Saudi follow the same pattern.

Interestingly this mirrors the pattern in many cities during the great age of Metro building in the western world (early 20th century) where many lines were elevated above streets. This didn't happen in London, probably because the streets were narrow and the locals would have objected strongly. Many were later buried, but traditional "elevated" survives in places like Chicago and even Paris and used to be extensive in New York.

A surface railway, and to some extent a viaduct, also creates something called "community severance" where it doesn't follow an existing transport route - basically it becomes a psychological barrier as well as a physical one and the areas either side become disconnected (literally "the wrong side of the tracks").

Geology also has an effect. Much of central and north London is on clay, which is easy to tunnel, but different geology in the south helps explain why few Tube lines go much beyond the Thames (Prevalance of suburban railways is another reason, whereas the northern railways had long-distance traffic to nurture so were less interested in commuters). Manhattan is mostly solid basalt, which makes building skyscrapers relatively easy but means subway lines traditionally had to be blasted rather than bored. This probably explains why many of them are built by cut and cover under streets, and often have four tracks allowing express service.

More recently new tunnelling techniques have evolved, making tunnelling through sand far from impossible though I don't know the details of how it is done. The latest subway line in New York is bored, as for obvious reasons are the tunnels under the rivers either side of Manhattan (although immersed tube was used for a similar situation in San Francisco). Recent lines in Barcelona have adopted a huge tunnelling shield, which allows a horizontal divider to be put in the resulting tunnel with one track above and one below. There is room to build platforms on any straight section without the expense of widening the tunnel, so it appears that the saving on station costs is enough to pay for a much bigger machine and volume of excavation than traditional double or single bores.
 

HowardGWR

Established Member
Joined
30 Jan 2013
Messages
4,983
Landscape is a visual concept. This definition was found.
"A landscape is the visible features of an area of land, its landforms and how they integrate with natural or man-made features."
You seem to be referring to geology in your examples. Granted geology is heavily determinant of landscape but see the above definition.
In the UK an AONB or NP are legal landscape definitions that impose planning restrictions. This is why HS2 is mostly in tunnel through the Chilterns AONB.
Noticeably, they managed to ignore that when the M40 was driven through. :frown:
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
In pure construction cost, building at ground level is a lot cheaper than building a viaduct, which is a lot cheaper than building a tunnel. But other factors such as land prices and existing buildings or roads mean the cheapest option isn't always possible, either for economic reasons or because the local authorities won't allow it.

Much of the Dubai Metro runs along Sheik Zayed Road, the country's main highway. It was relatively easy to find space for the piers of a viaduct but finding a strip wide enough for a surface route would have been much more difficult. Also there are frequent side roads so the Metro would have had to go over or under each one. All in all far easier just to put it on viaduct. The tunnels are near the older part of the city, which I haven't visited but where it is probably more difficult to carve a path through for a viaduct. Other metros under construction or planning in Saudi follow the same pattern.

Interestingly this mirrors the pattern in many cities during the great age of Metro building in the western world (early 20th century) where many lines were elevated above streets. This didn't happen in London, probably because the streets were narrow and the locals would have objected strongly. Many were later buried, but traditional "elevated" survives in places like Chicago and even Paris and used to be extensive in New York.

A surface railway, and to some extent a viaduct, also creates something called "community severance" where it doesn't follow an existing transport route - basically it becomes a psychological barrier as well as a physical one and the areas either side become disconnected (literally "the wrong side of the tracks").

Geology also has an effect. Much of central and north London is on clay, which is easy to tunnel, but different geology in the south helps explain why few Tube lines go much beyond the Thames (Prevalance of suburban railways is another reason, whereas the northern railways had long-distance traffic to nurture so were less interested in commuters). Manhattan is mostly solid basalt, which makes building skyscrapers relatively easy but means subway lines traditionally had to be blasted rather than bored. This probably explains why many of them are built by cut and cover under streets, and often have four tracks allowing express service.

More recently new tunnelling techniques have evolved, making tunnelling through sand far from impossible though I don't know the details of how it is done. The latest subway line in New York is bored, as for obvious reasons are the tunnels under the rivers either side of Manhattan (although immersed tube was used for a similar situation in San Francisco). Recent lines in Barcelona have adopted a huge tunnelling shield, which allows a horizontal divider to be put in the resulting tunnel with one track above and one below. There is room to build platforms on any straight section without the expense of widening the tunnel, so it appears that the saving on station costs is enough to pay for a much bigger machine and volume of excavation than traditional double or single bores.

You utter legend! That answer is brilliant! <:D
 

WatcherZero

Established Member
Joined
25 Feb 2010
Messages
10,272
I believe they tunnelled under the central business district as suggested because it was cheaper than demolition and not available land like along the road. They did have issues with the tunneling, it being sand helped in some respects as they used positive pressure to keep sand and water out until the walls could be placed, they could also inject the concrete in to the walls easier as it was permeable. What they did have issues with was the tunnel was inside the watertable and they were worried about subsidence above if the water emptied in to the tunnel. The large amount of water while drilling through sand caused a lot of heat to be produced and the steam was damaging the drill head blunting it so they had to add extra cooling equipment to the TBM, ironically being inside the water table they could just cool the water coming out of the ground then flush it back across the drilling head to cool it.
 

w1bbl3

Member
Joined
6 Mar 2011
Messages
325
Apologies. I'm no expert on geology. I should've just said landscape. But do you know if the landscape in Dubai affects the new metro? The majority of it is above ground elevated over street level, with a short section underground but in a single bore tunnel. Does the fact the city sits on sand have any effect on what tunnels can be built compared to boring through limestone or hard rock like in London where many tunnels can exist alongside, above and below each other?

The Dubai metro tunnels are EPB bored tunnels with modern methods you can bore a tunnel through the majority of ground types including sand. Sometimes it might be necessary to improve the ground ahead of the boring machine by grouting or ground freezing.

In terms of bored tunnels the basic choices are:
- open face, suitable for good clay ground or rock with no water
- Earth pressure balance, clay and slit with low water pressure
- Slurry Shield, clay, slit and sand. Possibly high water pressure
- Rock shield / double shield, rock, not stable.

Mostly new underground metro projects will use EPB or SS machines to keep ground disturbance around the bore to a minimum.

Coming back to the original question "green cities" such as say London, NY or Paris needed to introduce metro systems many years ago when the technology available to construct rail lines was less sophisticated than today, or indeed methods available only forty years ago.
As such the system builders favoured ground level or elevated constructions including if needed the purchase and demolition of existing properties to tunnelling. Only where land was not available commercially or politically was tunnelling used and then mostly shallow cut and cover tunnelling methods. These where quicker and cheaper to construct than manually excavated deep tunnels but had the consequence of being highly disruptive during construction as an existing street was excavated for the tunnel to go below.

The modern boring machine capable cutting the full bore is a 20th century invention first appearing in the 1950's, followed by pressured versions in the 60's and rock shield version's in the 70's, enabling boring to move from stable rock to clay, silt, sand and fractured rock with high ground water pressures. Contemporary metro system can take advantage of this to construct networks without needed to disrupt the everyday life of city residents and workers where budget is available.

The relatively unnoticeable progress of crossrail vs the years of disruption caused during the construction of tram networks in Manchester, Sheffield and Edinburgh, hilights why a subterranean metro may be preferable to city planners.
 

Up_Tilt_390

Member
Joined
10 Oct 2015
Messages
923
The Dubai metro tunnels are EPB bored tunnels with modern methods you can bore a tunnel through the majority of ground types including sand. Sometimes it might be necessary to improve the ground ahead of the boring machine by grouting or ground freezing.

In terms of bored tunnels the basic choices are:
- open face, suitable for good clay ground or rock with no water
- Earth pressure balance, clay and slit with low water pressure
- Slurry Shield, clay, slit and sand. Possibly high water pressure
- Rock shield / double shield, rock, not stable.

Mostly new underground metro projects will use EPB or SS machines to keep ground disturbance around the bore to a minimum.

Coming back to the original question "green cities" such as say London, NY or Paris needed to introduce metro systems many years ago when the technology available to construct rail lines was less sophisticated than today, or indeed methods available only forty years ago.
As such the system builders favoured ground level or elevated constructions including if needed the purchase and demolition of existing properties to tunnelling. Only where land was not available commercially or politically was tunnelling used and then mostly shallow cut and cover tunnelling methods. These where quicker and cheaper to construct than manually excavated deep tunnels but had the consequence of being highly disruptive during construction as an existing street was excavated for the tunnel to go below.

The modern boring machine capable cutting the full bore is a 20th century invention first appearing in the 1950's, followed by pressured versions in the 60's and rock shield version's in the 70's, enabling boring to move from stable rock to clay, silt, sand and fractured rock with high ground water pressures. Contemporary metro system can take advantage of this to construct networks without needed to disrupt the everyday life of city residents and workers where budget is available.

The relatively unnoticeable progress of crossrail vs the years of disruption caused during the construction of tram networks in Manchester, Sheffield and Edinburgh, hilights why a subterranean metro may be preferable to city planners.

Another great informative answer! <:D
 

Jonny

Established Member
Joined
10 Feb 2011
Messages
2,562
I think a lot comes down to the cost: tunnelling is expensive but more politically acceptable, and also saves on buying land. It only occurs rarely because there are few cases where the cost can be justified.

Also there is the politics of obtaining land for new-build lines at surface level; just look at the furore associated with the HS2 project.
 

Jonny

Established Member
Joined
10 Feb 2011
Messages
2,562
Then there is the matter of routing; it is necessary for there to be sufficient potential demand; however trains do not take well to steep gradients. Trains can achieve high speeds on level ground with relatively low power-to-weight ratios, thanks to the low friction steel-on-steel nature of the railway. However difficulty can arise long before the gradient-related loss of traction becomes a factor, as this is soon offset by the need to convert to gravitational potential energy in order to travel uphill. A typical power/weight ratio will be about 6 kW/tonne for a second generation multiple unit (British 14x/15x) to maybe 10kW/tonne for high performance conventional speed rolling stock and rising to maybe 20 kw/tonne for TGV-type trains.

Motor vehicles such as private cars on the other hand will easily turn out 50 to 100kW/tonne, which gives them superior hill-climbing ability aided by the additional grip that comes from rubber-on-tarmac. This is necessary to ascend a steep hill at a "good" speed. Unfortunately, rubber-on-tarmac is notoriously inefficient due to the high rolling resistance from the interface. Given that steel-on-steel will likely have adhesion issues at this level even in ideal (dry and clean) conditions, railway designers have to factor in whether to build a tunnel/cuttings, build embankments/bridges/viaducts or a combination thereof, go around (or some combination of the three) and that's assuming that the height differential associated with the journey is not too great for a railway to be a viable proposition.
 

edwin_m

Veteran Member
Joined
21 Apr 2013
Messages
24,793
Location
Nottingham
Then there is the matter of routing; it is necessary for there to be sufficient potential demand; however trains do not take well to steep gradients. Trains can achieve high speeds on level ground with relatively low power-to-weight ratios, thanks to the low friction steel-on-steel nature of the railway. However difficulty can arise long before the gradient-related loss of traction becomes a factor, as this is soon offset by the need to convert to gravitational potential energy in order to travel uphill. A typical power/weight ratio will be about 6 kW/tonne for a second generation multiple unit (British 14x/15x) to maybe 10kW/tonne for high performance conventional speed rolling stock and rising to maybe 20 kw/tonne for TGV-type trains.

Motor vehicles such as private cars on the other hand will easily turn out 50 to 100kW/tonne, which gives them superior hill-climbing ability aided by the additional grip that comes from rubber-on-tarmac. This is necessary to ascend a steep hill at a "good" speed. Unfortunately, rubber-on-tarmac is notoriously inefficient due to the high rolling resistance from the interface. Given that steel-on-steel will likely have adhesion issues at this level even in ideal (dry and clean) conditions, railway designers have to factor in whether to build a tunnel/cuttings, build embankments/bridges/viaducts or a combination thereof, go around (or some combination of the three) and that's assuming that the height differential associated with the journey is not too great for a railway to be a viable proposition.
The maximum gradient compatible a particular technology is related to its maximum acceleration and braking rate, as both depend on the coefficient of friction. For steel wheel systems both figures are about 10% in the extreme case with all axles motored (eg Sheffield Supertram) and more typically 3-5% for maximum gradient and 8-10% for acceleration and braking.

I'd actually argue that this is also about the maximum figure that a rapid transit system should be using in any case, as greater acceleration and braking starts becoming dangerous to standing passengers. The rubber-tyre trains on the Paris Metro carry warning signs about severe braking and you are likely to fall if not holding on to a handhold when the brakes go on. The same applies to buses, with the added hazard of severe cornering rather than the more gentle curving enforced when the vehicle is on rails. Between that and the extra rolling resistance I suspect the French regret going for rubber-tyre technology for a full-size Metro, although they probably won't admit it. Modern systems such as Dubai have favoured steel wheel.

As mentioned, electrification is also a key parameter. Trains serving underground stations moreorless have to be electric - the Metropolitan tried it with steam in the 1860s but electrified pretty much as soon as the technology was available. Viable electric traction also made long bored tube tunnels possible, although a few early systems tried cable haulage. Most modern metros choose a third rail, but a bottom contact system which is safer than the top contact used in older systems. This is workable at the lower voltages which are optimum for intensive metro routes (mostly 750V but a few 1500V third rail systems exist), and compared with an overhead line it reduces the cross-section of tunnels and the visual impact of viaducts. However for mixed traffic railways the economics favour a higher voltage, which can only be supplied by overhead line.

Generally railways are electrified when they need to run more and longer trains, so that the lower running costs of the electric trains outweigh the cost of building and maintaining the electrification equipment. So it's probable that railways in low-population areas will not be electrified as passenger demand will be low, but there are exceptions. In particular some heavy haul railways, largely or totally dedicated to the haulage of very heavy freight usually from a mine to a port, have adopted electric traction. As for electrified railways in desert, a high speed line is being opened between Makkah (Mecca) and Medina.
 

Bantamzen

Established Member
Joined
4 Dec 2013
Messages
9,680
Location
Baildon, West Yorkshire
On the subject of the NYC Subway, it is worth noting that 40% of the network is above ground, mainly on elevated tracks often built over existing roads. Much of the remaining sub-surface lines were constructed by cut and cover methods. These methods were deployed because of both geology, as the area was former marshlands and so the ground is soft and gravelly, and facilitated building in limited spaces in a rapidly expanding city.
 
Status
Not open for further replies.

Top