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Comment by IIAOPSW | Hacker News Reader

Or, here's the real bold one, build underground but use cut and cover rather than deep tunnel boring machines that only a few international consortiums know how to run. 1. cut up street. 2. lay down tracks. 3. cover up with viaduct but pretend its a normal road.

This is how NY originally built the subway and it has so many advantages. Can make use of any local unskilled labor. Cheaper by a lot. Stations are close to surface, not a labyrinth of escalators to hell. You won't have street level tracks cutting the town in half. Its the superior option in literally every way, but it requires making people put up with their street being in a constant state of roadwork while its built. No one has the tenacity to tell certain voters to just put up with things for the greater good anymore. Its easier to spend a billion dollars than do that!

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Cut and cover isn't as simple as it used to be because of the huge amount of underground infrastructure laid down over time, without proper mapping. Digging up any street in NYC is labor intensive, time consuming, and dangerous.

Whether it would still be less expensive than the alternative very deep tunnels, that I do not know.

Only for the record, there is a midway solution.

Basically you go through a street making two walls on the two sides, using either piles or diaphragms, then you excavate the whole width and build a concrete slab resting on the walls and restore the street surface, reopening the street/road to normal use.

Then you excavate under the slab (that becomes the tunnel cover) and between the walls.

This technique is fastish and safe, but as someone else commented in modern cities the amount of services (cables/pipes/etc.) you will find and need to modify, move, etc. may create really huge costs and use a lot of time.

Cut and cover is extremely disruptive to the residents around where you're cutting, and extreme care needs to be taken to divert any critical infrastructure (water, electricity, internet, etc.) in the way. So it won't necessarily be cheaper, and will absolutely involve a lot of NIMBYs with legitimate complaints (who wants to spend a year next to an active construction site?).

Tunnel boring machines aren't that expensive per km. They are robotic worms that take a few dozen people to operate. If you do cut and cover you will end up with a similar problem to the tram system in Edinburgh where there were a lot of pipes and cables under the street that were uncharted and not as deep as everyone expected. Dealing with old pipes and cables is very delicate; it involves a lot of careful hand digging with shovels and picks and then careful propping to avoid damaging the old clay drain pipe / ancient electricity cable / iron gas main / water main etc. All of these can be lethal if they are damaged and they all have to be worked on by hand. Super expensive.

The half and half solution (which IIRC the report suggested) is cut and cover stations with bores between stations. Digging out stations from the inside out is what's really insanely expensive.

When NY originally built the subway, labor laws were a lot more lax, and labor was much, much cheaper.

True, but we have plenty of examples around the world showing that cut and cover us cheaper. We have contractors in NYC who dig holes all the time, cut and cover just a long hole and so easy to estimate.

Also it was a lot easier to evict people and tear down anything standing in the way.

> Stations are close to surface, not a labyrinth of escalators to hell

Seattle is building an extension to its light rail system partially above ground partially below ground in many places. It chose to build like this intentionally. Not saying I agree with it but the reasoning:

You enter from any side of street and go to unified mezzanine level. From there you can go to both sides of tracks. This enables easier handicap/elevator access to relevant areas since You only need one elevator to street level. Additionally, no signage on street level to navigate to delay anyone. You enter the station (from anywhere) then figure it out inside without worrying about crossing street to find track.

This is the most energy efficient layout. Gravity will slow trains uphill into the station and then give a boost downhill when leaving. It’s not always possible though.

its only more efficient when more people are boarding than leaving.

Sorry. It’s not a station that’s partially above and below. Some stations are above ground, some stations are below ground. All stations are three levels: ground + mezzanine + track.

Seattle is very hilly so keeping the tracks at shallow grade changes causes the track to be above ground in some area and underground in others.

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