
Cooling the tube – Engineering heat out of the Underground - mzehrer
https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-tube-engineering-heat-out-of-the-underground/
======
kator
Subways in NYC are not fun in the summer either. I always assumed it was
because when they were designed they didn't consider a future state where air
conditioners on the trains dump their heat into the tunnel.

I tried searching for a similar study for NYC but all I found was old articles
from years back.

It doesn't look like the MTA shares any measurements of temperature in their
data feeds:
[http://web.mta.info/developers/download.html](http://web.mta.info/developers/download.html)

Does anyone have ideas on how we could get this sort of data for NYC subways?

~~~
ice109
there is no way that the NYC Subway's AC's dumping heat into the tunnels
contributes in any meaningful way to the temperature inside the stations - the
tunnel system is enormous relative to the trains. much more it's simply NYC
summers are sweltering and the stations aren't well ventilated at all.

~~~
CydeWeys
Did you read the linked article? The heat of the London Underground tube
tunnels is directly caused by heat released by trains (though primarily from
brakes and friction, not AC), and has nothing to do with season. It's
swelteringly hot down there in winter.

I don't see why it'd be any different for NYC, where I live.

~~~
pvg
Many NYC subway tunnels are not 'tubes' and are shallow enough to have surface
access. So part of the answer is probably that the NYC subway is a more open
system.

~~~
CydeWeys
The same is true of London (also mentioned in the linked article). The linked
article is explicitly only talking about the deep tunnels, which NYC also has.

~~~
pvg
The point is there are more shallow, cut-and-cover tunnels in NYC than in
London. London has entire high-traffic lines that are deep tunnels.

~~~
jrockway
Yeah, NYC is a very different system. The deep tunnels are very well
ventilated. Go to a station at the end of a deep section, like High Street and
you can feel the wind blowing out because of the forced-air ventilation
system. You can also see daylight from the subway trains throughout the
system, because it's that close to street level and there are ventilation
grates every hundred-ish feet.

------
Jyaif
The Montreal subway system has a very clever way of somewhat saving energy
(and emitting less heat): the section of the track at the station stop is
higher than the rest of the track. This means that the train's kinetic energy
is converted to/from potential energy whenever the train arrives/leaves the
station stops.

~~~
ant6n
I feel like in the long term, this may become a liability compared to
regenerative braking and level tracks.

For example the Montreal metro currently only allows a train to leave a
station when the platform of the next one is free, limiting the frequency of
this very crowded system. With modern signaling, trains can creep up to the
ones before them and reduce the time between them down to 40 seconds - but
it's more difficult with all those slopes around.

It's also making extending platform lengths or moving/adding stations nearly
impossible.

~~~
Joakal
In Australia when it's flat, there were still collisions. Trains are very very
heavy. The collision report recommendations had been to keep a long distance
between trains. For trains ahead, the coming train would slow down and would
come to a stop, if it's probably one train length ahead.

I'm not sure how long the distance is though. 40 seconds is really really hard
to estimate with very very heavy trains and no weight sensors. Even AI/ML
cannot predict this (re: no weight sensors).

~~~
CountSessine
Why would a computer need weight sensors? It knows exactly how hard the
electric motor worked when it was accelerating the train and exactly how
quickly the train accelerated. It should be able to come up with an entirely
usable estimate of the train's weight.

~~~
ant6n
Just as an aside, modern transit trains have weight sensors anyway which
adjusts the pressure in the air suspension to make sure the train is exactly
level with the platforms.

------
f_allwein
Related: tube map showing temperatures [http://www.gizmodo.co.uk/2017/05/its-
official-the-bakerloo-i...](http://www.gizmodo.co.uk/2017/05/its-official-the-
bakerloo-is-the-hottest-london-underground-line/)

~~~
kevin_thibedeau
So two lines just barely break 30C for a couple months in the year. Not
exactly a crisis.

~~~
estel
I believe those are temperatures in the actual tunnels, a passenger's
experience in a crowded tube car will be quite a bit warmer!

~~~
bb611
Underground cars don't have AC/heat? At least in the US those are ubiquitous
on subways

~~~
matthewmacleod
The deep level London tube lines would be difficult to fit AC units to given
the small size and tight tolerances.

Newer units on the subsurface lines do have AC fitted.

~~~
Havoc
>The deep level London tube lines would be difficult to fit AC units to given
the small size and tight tolerances.

Mines quite successfully manage to cool air 2+ miles under ground, so I'm
finding it a rather hard to believe that nothing can be done about this.

~~~
sithadmin
I think they're referring to cooling of the train cars.

The deeper tube cars running on older lines have serious space constraints to
deal with that make integration of things like cooling equipment difficult and
expensive. We're talking about trains that have comically small proportions
relative to what you would see anywhere else in the world.

That said, last I had read, the deep line cars are supposed to get AC sometime
within the next decade or so.

~~~
Havoc
>I think they're referring to cooling of the train cars.

Don't think so. That is a fixable problem. (e.g. convert the space of 1 seat
per car for an AC unit).

The fact that everyone is so stumped by the problem says it's the actual
tunnels that are overheating. The fact that some lines don't have AC just
exasperates this.

------
f_allwein
> offered a prize of £100,000 to anyone who could come up with fresh ideas

Too late now, but I wonder if they considered district cooling, where e.g.
cold water from rivers is used as an alternative to air conditioning. Seems to
be used successfully in my hometown of Munich:
[https://www.swm.de/english/m-fernwaerme/m-fernkaelte.html](https://www.swm.de/english/m-fernwaerme/m-fernkaelte.html)

~~~
avianlyric
They have used water cooling from rivers in some stations (I can't find the
source, but I think it was a previous ianvisits post).

Unfortunately only a couple of stations have an appropriate water supply, and
enough free vertical shaft space to fit the pipes.

~~~
twic
There are some details on that in the Ian Visits article linked to in another
comment.

There was also this, from 1938, although i don't know where they got the
water:

[https://en.wikipedia.org/wiki/Tottenham_Court_Road_chiller](https://en.wikipedia.org/wiki/Tottenham_Court_Road_chiller)

------
franciscop
The Tokyo (and Japan in general) underground/subway is actually quite fresh
and amazing in the hot summer. How do they do it? It might be interesting to
learn from them and a good question for the Underground of London Engineers.

~~~
eru
Singapore's system is also airconditioned. They just engineered it properly
when they built it.

~~~
lostlogin
In fairness the London Underground is from the 1860s, Singapores if from the
1980s. That said, London's system was designed for coal, so imagine how bad
the air must have been back then.

~~~
krallja
As the article states, there are two distinct types of underground rail in
London: the older sub-surface lines, and the newer tunnels. The coal trains
were never used in the tunnels.

~~~
nickkell
I'm not sure what the difference is between the two. Aren't they both
underground? Here's a video of a steam train going thru the underground to
celebrate 150 years of the tube:
[https://www.youtube.com/watch?v=a3_rxuOFTm8](https://www.youtube.com/watch?v=a3_rxuOFTm8)

~~~
Mvandenbergh
The sub-surface lines were built by digging trenches and then covering parts
of them back up with roads, buildings, etc. This technique is called cut and
cover. They're just beneath the surface and large stretches of them are
actually open to the air, many were built for coal powered trains. These can
be built without tunnelling. They're below-ground but not usually "under
ground". The trains are rectangular shapes since that is the traditional shape
for a train.

The deep lines (originally called "the tube" although that now refers to the
whole system) were built by tunnelling, the trains and tunnels are cylindrical
and the trains just fit into them. In many cases the tunnels are 30+ metres
underground.

------
Animats
Long-term heat buildup was known in the design stage to be a problem for
Eurotunnel.[1] Huge chilled water plants were built to prevent that from
happening. It's a surprise, though, that it would be a problem for the London
Tube, which has so many connections to the surface.

[1] [http://www.nytimes.com/1991/05/01/business/business-
technolo...](http://www.nytimes.com/1991/05/01/business/business-technology-
air-conditioning-a-32-mile-tunnel.html)

------
raverbashing
How much would it cost to add regenerative braking to the cars?

And instead of ventilation shafts you would probably need active heat pumps

~~~
zkms
Per [https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-
tube...](https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-tube-
engineering-heat-out-of-the-underground/) there already is regenerative
braking on some cars, the issue is that regenerative braking can't happen if
there's no train on the same section of DC bus that can _accept_ the power.
There needs to be some sort of inverter to sink the higher DC voltage and send
it back into the grid.

~~~
robryk
Or you can just convert that energy into heat, but do that on the surface.
Have large resistor banks on the surface that you connect to the DC grid when
the voltage is too high.

~~~
hydrogen18
That would work, but a large flywheel would also be a good solution. You could
spin up the flywheel to store energy and if it reaches maximum speed then use
the resistors. You'd also need to detect load on the grid and then run the
flywheel system in reverse to assist vehicles that are moving.

~~~
late2part
"run the flywheel system in reverse to assist vehicles that are moving"

Just to be clear, you wouldn't actually run the flywheel in the opposite
direction.. You'd take energy out of the flywheel versus putting it in?

~~~
CydeWeys
Correct.

------
avar
I don't understand why lack of space above ground is a hindrance to building
new ventilation shafts. Surely these aren't going to be wider than a sidewalk,
and in central London the distance between any two roads on a block is rarely
more than 50-100 meters.

You'd end up with lots of ventilation grates on the sidewalks on the surface,
but that seems like an easy and space efficient solution.

~~~
eponeponepon
A substantial part of the problem is the sheer age of central London; people
have been digging beneath it and piling more buildings on top of it for nearly
2000 years, so given any particular spot in the Tube network, there's every
chance that if you try and drill upward from it, you'll hit part of the sewer
system, a buried river, someone's wine cellar, something top-secret belonging
to the state, a lost graveyard, a plague-pit...

~~~
pbhjpbhj
... utilities too. There are probably not many spaces a vertical shaft could
go.

You probably don't want a direct vertical grate either, you'll need to catch
water, rubbish, people from falling down. An S-bend is going to effect flow.

------
tomohawk
I wonder if using a different means of regenerative breaking would work, such
as hydraulic hybrid.

[https://en.wikipedia.org/wiki/Hydraulic_hybrid_vehicle](https://en.wikipedia.org/wiki/Hydraulic_hybrid_vehicle)

Much more of the power would be preserved, leading to less heat.

------
richardjennings
Is it feasible to increase the distance into surrounding clay that heat can be
conducted? Could metal be used to conduct heat from clay surrounding tunnels
to clay that is presumably cooler, further away? I could imagine that if a
material exists that insulates electricity but conducts heat very well, the
track itself might be useful in transferring heat to cooler clay, making the
track colder and cooling tunnels.

~~~
inetknght
...and when _that_ clay heats up?

------
GoodAdmiral
It's quite incredible that the clay soil is still absorbing heat from the
introduction of the tunnels. I heard possibly wrongly that the clay drys out
too and ends up insulating the tube lines over decades.

Regenerative breaking sounds like the quickest and cheapest way to address the
problems - not that any change would be 'quick' or 'cheap'.

------
Dwolb
What about a mechanical sling shot system at each sub station?

As the train arrived it gets slowed by a spring or similar system which is
then used to propel the train forward once it needs to depart.

Also, what about just slower trains? The heat produced while acccelerating or
braking is probably not exactly linear with the speed of the train.

~~~
jdmichal
It would probably be simpler to use regenerative braking like a hybrid car.
The trains are already electric.

~~~
cjrp
It says in the article they convert some of the heat caused by braking, but I
thought regenerative braking meant you also didn't have to brake as much.

~~~
jdmichal
Your don't have to engage friction-based brakes as much, yes. And since
friction-based brakes work by converting movement to heat, that's why
regenerative braking helps with heat.

------
amoorthy
Interesting article. However I can't recall tube stations being much warmer
than outside temperatures in winter (on non-windy days). If that's right then
how is the heat dissipated better in winter?

I don't live in London so anyone with regular riding experience please correct
me if I'm wrong.

~~~
an_account
I was wondering that too. If they clay is able to cool off and "reset" in
winter then the real story is just that these lines see more use today.

~~~
robotresearcher
The tube is warm in winter. It would be nice except you are typically wearing
a winter coat so it's hard to get it just right.

------
ricw
I never got why the heat in the tube was not being used as a heat source. You
could extract the heat and supply it to surrounding buildings at a cost,
thereby cooling the tube. The tech is readily available. It would be a win win
situation. Plus it'd be be very environmentally friendly.

~~~
CydeWeys
The problem is that the heat isn't a point source, it's diffused across
hundreds of kilometers of tunnel. The amount of infrastructure you'd have to
build to extract it at scale would simply cost way too much. It's cheaper for
any reasonable timeframe to simply continue burning natural gas at the surface
for heat than to invest all of this into infrastructure at a very long-term
ROI.

~~~
eru
Thermodynamics also makes this really hard. The tube is at most 40 degrees
celsius. The air outside is perhaps 20 degrees celsius in summer. There's just
not enough of a temperature gradient to run any kind of efficient engine.

Natural gas burns at around 2000 degrees celsius.

~~~
dredmorbius
It could be useful for low-grade thermal applications -- heating (or pre-
heating) hot-water supplies, or space heat.

Even simply ducting warm air to street level for outdoor dining (in winter),
presuming (against other information, I'm aware) that the ducting could be
provided. If you're wasting the heat _anyway_ , put it to _some_ use prior to
final venting.

------
toyg
> _The future of the cooling the tube project will be judged not so much by
> how they cool the hot tunnels, but by how they stop tunnels becoming hot in
> the first place._

That's a very good metaphor for our planet.

------
jpalomaki
"Over 47 million litres water are pumped from the Tube each day" [1]

Use the heat from air in tube to warm the water that is anyways getting pumped
up. (Too lazy to do the math to see if this would make any difference)

[1]
[http://www.telegraph.co.uk/travel/destinations/europe/united...](http://www.telegraph.co.uk/travel/destinations/europe/united-
kingdom/england/london/articles/London-Underground-150-fascinating-Tube-
facts/)

------
l5870uoo9y
Is the temperature still rising or have it plateaued?

~~~
rwmj
And how long is it "stored" in the ground? If they closed the tube for a week
_(I know)_ would the earth return to 14C?

~~~
TheOtherHobbes
No. I doubt anything less than a decade would make much of a difference.

It's taken a century or so to raise the temperature in the tunnels by around
ten degrees C. That _already includes the cooling effect of cold air being
pushed into the tunnels for most of the year_ , balanced by the relatively
small number of days a year when the air temp in London is more than 14C.

Without that cooling heat has nowhere much to go. It will radiate out into the
air, which will make its way up and out rather slowly. And it will diffuse
into the clay/soil around the tunnels, even more slowly.

A fully passive cooling-off period would take years - at least.

The problem isn't impossible to solve. All kinds of active cooling solutions
are possible.

The problem is that it's impossible to solve _affordably._ You effectively
have to build a heat exchanger the size of central London, which is never
going to be cheap.

------
dredmorbius
The general strategy would be a) introduce less heat and b) extract more. The
first might be accomplished through greater efficiencies, though that's
limited.

If a major heat component is braking, then locating the additional cooling
capacity where breaking is heaviest (presumably on inbound station approaches)
might offer advantages -- at the very least this reduces the total treated
area for maximum effect.

Given the possiblity of ground-based thermal banking, and the long-term nature
of the issue, if _any_ amount of coolant could be circulated through the
thermally-affected clay, and made available for seasonal heating needs
elsewhere in the city, that might be a net win.

I'm familiar with geothermal energy projects elsewhere (borehole projects in
Australia, the Habernero project) where the problem is actually inverted:
themal _extraction_ cools the strata around a borehole, over the course of
about 40-50 years, to the point that no further useful heat can be extracted.

The thought also occurs that the steel rails themselves are thermally
conductive and might be made a part of the cooling system. Not a tremendous
radiative surface, but a long conductive length. Poorly placed, that is, low
within the tunnel, rather than high, for effective heat extraction though.

------
ziikutv
Silly question, is the heat so low that it cannot be used for something other
than releasing above ground?

~~~
eru
It's only about 40 degrees at most. So the answer is basically, yes.

------
memracom
Here is an idea that I sent to the Underground in 2006 when they solicited
suggestions from the public.

Add cool to the tunnels, rather than taking heat out.

Build liquid air plants above ground, 2 or 3 floors up in the air so that the
heat of the pumps is released above street level and the noise can be kept
away from the street. Feed the liquid air into the tube tunnels through
insulated pipes which takes up far less volume than air vents. Let gravity
bring the liquid air down the pipes. Release the liquid into the tunnels near
platforms where the air pump effect of moving trains caused lots of air
circulation. Also the car doors open on the platforms.

Since you are liquifying the air, not just the oxygen, it can be safely
released anywhere in the tunnels. And if your air intakes are high up you will
actually be improving the air quality in the tunnels as well, i.e. cleaner air
flows in.

~~~
wbl
This is a terrible idea. First lets ignore the thermodynamic efficiency losses
compared to a regular chilled water cooling system. Liquid air is a potent
oxidizer and the Underground has myriad sources of ignition and flammable
things like people. Plus the risk of frostbite from exposure is very real. A
control failure could kill people and ignite a raging fire underground.

~~~
memracom
Liquid air is the same as normal air. During its liquid stage it would be
entirely enclosed in pipes. It can be safely expanded using the technology
used in mine rescue suits like those from Draegerman.

You are confusing it with liquid oxygen which is just as dangerous in its gas
form as it is when liquid. Air has some 70+ percent nitrogen in it, whether
gas or liquid, and that prevents it from being any more corrosive or flammable
than plain air.

------
mixedmath
I wonder, how long would they need to shut down the tube before temperatures
lowered?

~~~
BillinghamJ
Years, possibly even decades. The problem is that the clay just doesn’t really
release the heat - it’s very good at retaining it.

------
Tharkun
How much heat are we talking about? I'm guessing it wouldn't be enough to use
for district heating, the way some industrial waste heat is converted to hot
water for homes?

~~~
manmal
The original source [1] has a graphic at the bottom where it says about 300m
kWh per year currently, so about 34MW on average if I didn't miscalculate.

1: [https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-
tube...](https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-tube-
engineering-heat-out-of-the-underground/)

------
sjg007
Also if 50% of the heat is from braking and accelerating at the platform, why
not just AC the crap out of each station?

------
sambull
Long term heat build up because of human activities. It may not because of
climate change, but this is how humans change climates.

~~~
ams6110
The actual heat released by human activity is not significant in climate
change.

------
Roritharr
What is so "experimental" about the air coolers in the picture? They look like
normal A/Cs

~~~
pjc50
The tube did not have powered A/C until fairly recently - it was built with
ambient air ventilation only.

~~~
avianlyric
And that A/C is only on cut and cover lines (circle etc) not deep level
trains.

On deep level trains there is no good place for the trains to dump the heat.

------
sjg007
I'd look at how ants or termites cool their nests for inspiration.

~~~
dredmorbius
By not running trains through them, for starters ;-)

------
zkms
> Most of the tube tunnels have above ground sections, so a hybrid idea is to
> use air conditioning in the trains when above ground, and while above ground
> to cool a block of “phase change media”, or water to you and me, into an ice
> pack. When underground, the heat that would be dumped in the tunnels is
> absorbed by the ice-pack until it has returned to water.

> Whether this can be viable is still being looked at, bearing in mind that
> they already struggle to fit air conditioning units into tube trains,
> finding space for the ice blocks is going to be even more of a headache. And
> not to forget that the extra weight means more energy needed to drive the
> trains, driving up running costs.

This can be worked around, _do the chilling on the wayside_ , not on the
train! At each station, run chillers that can reject waste heat on the ground
-- and chill a nontoxic liquid glycol/water mixture to -40 C. Commercial
equipment exists to do this already. Have air/liquid heat exchangers on each
EMU, along with glycol storage tanks, a pump, and sensors to keep track of the
temperature/volume of the glycol in each tank. On the roof of the EMU, install
large-diameter quick-mating liquid connectors, along with fiducial marks. At
each station, wayside equipment uses computer vision to locate the fiducials
on the EMU, mates with the connectors, does a pressure test to verify the
integrity of the connection (squirting glycol is a no-no), pumps out all the
warmed glycol (and replaces it with cold stuff), and disconnects. This can be
done during the dwell time if the connectors and refill tubing are of large
diameter.

Cooling loads are on the order of 50 kilowatts per EMU and inter-station times
are on the order of 10 minutes, which means 30MJ per EMU. The ending
temperature of the glycol will be on the order of 10C (you need a temperature
differential to ensure heat flows from the glycol to the air), its initial
temperature will be -40C -- a temperature difference of 50K. Glycol/water
mixtures have a specific heat of around 3.2kJ/(kg * K), so we have:

30 MJ = (3.2 kJ / kg * K) * mass * 50 K

leading to a mass on the order of 200 kg, which is quite tolerable for a rail
vehicle. The tanks for the glycol can be spread around the car and can be
arbitrarily shaped (as long as fluid can be circulated and offloaded) to deal
with other constraints. There's no phase changes involved, which makes the
heat exchange work non-annoying; there's just liquid glycol and air. EMUs
don't need to haul around an air-cooled chiller, all the equipment on the EMU
is reliable, does not consume much electricity, and is extremely tolerant of
vibration and the harsh environmental conditions aboard a rail vehicle.

If you want to reduce mass further and are willing to accept some more
complexity, it might be sensical to use a small chiller on each EMU that _uses
the glycol for heat-rejection_. What does this give you? It means that you can
still generate a constant chilled water temperature of 10C, but let the end
temperature of the glycol go _above_ 10C -- and more temperature range on the
heat storage fluid means more heat energy can be dumped into it. When the
glycol temperature gets above 10C, turn on the heat pump to create chilled
water at 10C, and reject heat into the glycol (stop before it boils).
Liquid/refrigerant heat exchangers are much smaller than air/refrigerant
exchangers, so if your compressor isn't obscenely heavy, you can likely save
some weight. If you can use the glycol from 10C (when heat won't passively
flow from the glycol to the air) to 60C (a reasonable condenser temperature)
-- that's another 50K worth of temperature difference, which means our 200kg
load of glycol can be cut in half.

------
iamflimflam1
More information available from the article's source:
[https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-
tube...](https://www.ianvisits.co.uk/blog/2017/06/10/cooling-the-tube-
engineering-heat-out-of-the-underground/)

~~~
hengheng
These are the first trains that I hear of with no regenerative braking.

~~~
crote
Correct me if i'm wrong, but it seems that regenerative braking is a bit
troublesome because it is a third rail direct current system: a "regular" AC
system can simply feed power back through the transformers to the power grid,
but this is not possible here, so power must be consumed by another train fed
by the same rectifier.

~~~
hydrogen18
It would be much more difficult to feed power back into an AC grid from a
regenerative brake. With a DC grid, no synchronization is needed.

~~~
F_r_k
That's not true.

1) modern railway is fully IGBT powered. In this case it is trivial to inject
current.

2) with DC current you need a substation capable of converting AC to DC (easy:
bridge rectifier) but also DC to AC (to given tolerances) which is much more
cumbersome.

------
eecc
For one thing I'm surprised they're not using regenerative brakes. It sure
will cost some of the profit but refurbishing the trains with these will cut
somewhat on that 80% of heat

~~~
lucaspiller
> It sure will cost some of the profit

TfL isn't a private organisation, they are a government body. In 2015/16 only
half of their costs were covered by ticket sales and other income
(advertising, sponsorships, etc). The rest comes from government funding, so
taxes.

~~~
Brakenshire
That's actually mostly not true any more. By next year day to day costs will
be 100% self-funded, only capital investment will be funded by the state.

[http://www.mayorwatch.co.uk/govt-funding-changes-will-
force-...](http://www.mayorwatch.co.uk/govt-funding-changes-will-force-tfl-to-
become-self-funding-by-2018/)

------
em3rgent0rdr
The surrounding earth acts as a temperature storage buffer, so without
ventilation, it will be extremely difficult to remove heat. Instead of the
more difficult problem of trying to _remove heat_ , I would rather focus on
simply _reducing the total heat produced_ in the lower levels. That can be
done by having the lower trains be only for express, so they aren't
accelerating and decelerating as much (which is when the heat is produced),
with fewer trains in general, and then save the tracks closer to the surface
focus for the more heat-producing non-express trains and for the more frequent
trains.

On a related subject, the temperature of any cave will remain almost constant
at the location's average annual surface temperature. So another option would
be to focus on not producing so much heat in the entire city in the first
place, to lower the average temperature.

~~~
vertex-four
> That can be done by having the lower trains be only for express

You have no idea of the layout of the Underground, obviously. They're not
lower trains that follow the same routes - they're entirely different (and
very important!) routes which just happen to be a lot lower than some others.

~~~
pjc50
Indeed - and the same line varies in depth across the city. Generally lines
are deeper closer to the core, and the Northern Line is anomalously deep due
to being built in 1860 when soil physics was not so well understood so extra
margin had to be allowed for the river.

Also, there isn't really such a thing as an "express" on the underground.

------
marze
The heat from the trains etc. heats the surrounding clay over a period of
years. You can add less heat, or you can remove heat from the surrounding
clay, or both.

Removing:

Run ventilation on high during winter and keep temps quite low in the tunnels.

Run ventilation on high on cooler nights.

Install cooling tubes in the surrounding clay and cool it directly. Either
from above, or from the tunnel itself, a ground-source heat pump (geothermal
heat pump) to pull heat from the clay. These can be powered by the cheapest
available power, likely solar on sunny days in the future.

Adding less:

Upgrade motors to highest efficiency available. This could halve the waste
heat from the motors.

Regenerative braking: if it is too complex to put the power back on the grid,
build large "electric kettles" and dump it into a vat of water with resistance
heaters. The water vat could be part of a water main so it would be constantly
refreshed, and result in slightly warmer water for water users.

Instead of ice in the cars, cool brakes and motors that exceed 100C with
water, by boiling the water. This absorbs terrific amounts of heat per kg
water.

~~~
someguydave
>cool brakes and motors that exceed 100C with water, by boiling the water.

While this would cool the trains and solve the heat problem over the long
term, I doubt most passengers would think that being sprayed with boiling
water from arriving trains (and the resulting damp) would improve perceptions
of traveling comfort

