Tube trains could navigate the Underground using the rules of Quantum Physics | Hacker News Reader

Tube trains could navigate the Underground using the rules of Quantum Physics

ianvisits.co.uk

36 pointsbeardyw79 comments

The Elixabeth Line runs 24 trains per hour automatically through a 10-station tunnel in central London. It mainly uses axle counters to measure where the train is.

https://www.railengineer.co.uk/controlling-the-elizabeth-lin...

> The signalling is essentially hands off and is timetable driven. The latter includes GoA3 reversing moves at Paddington and Abbey Wood which are fully automatic as are all entries into and out of a depot. The line uses axle counters for secondary train positioning information other than where neutral sections for the overhead power exist, where track circuits are deployed, these being seen as less vulnerable to any spark interference from the overhead catenaries.

Those odometers would drift as well with wheelslip, unless they are reset any time they enter a known section.

But it’s not an odometer. It’s a sensor on the track that counts axles. The system knows the precise location of the sensor, and how far axles are spaced.

That's interesting. Do you know if they are used to keep track of the trains' positions with axle-spacing precision everywhere, or only at stations and track-section boundaries? (my somewhat cursory search suggested probably the latter.)

I don't know for certain, but they'd have to have at least one set on both tracks in each ventilation section, to enforce ventilation rules (read the rest of the article about the tunnel's ventilation management). It also points out that axle counters can sense backward movements, should a stuck train require the ones following it to reverse out of the tunnel

It's kind of annoying that they don't go into specifics about why this is good. The problem with traditional accelerometers is that the error accumulates, and so even small errors accumulate. The article doesn't really address what makes this different - and in fact I don't think it is different. You're still just measuring acceleration using a sensor and that sensor will have errors that accumulate. So the question is how much better is it?

I would wager that actually this is probably just a way of funnelling money into research around quantum rather than genuinely trying to solve this specific problem. This specific problem sounds like it could be solved for a lot less money using conventional accelerometers in combination with some other local location data (optical sensors for example, you're in a very controlled environment).

> I would wager that actually this is probably just a way of funnelling money into research around quantum rather than genuinely trying to solve this specific problem.

You’re absolutely correct, from the government’s perspective the interest in the technology is for high accuracy inertial navigation systems for defence purposes, not for the London Tube. If you look at the other companies involved in this project, there’s a number of defence contractors involved.

This project isn’t really new, and historically the pitch has always been: We want to develop GPS grade navigation that doesn’t depend on satellites, and is smaller and better existing inertial navigation units. Oh look the London Underground is the perfect test bed for our technology!

It’s underground, so no GPS or many external signals. It’s already well mapped so we have something to compare against. Tube trains are loud, hot and vibrant a lot, which makes it a challenging environment for inertial systems. Plus it’s cheap and very easy to roll a box on an existing train, drive a few km under the city, and then compare your results to GPS from when you go underground, to when you surface again.

https://www.theguardian.com/science/article/2024/jun/15/lond...

The idea of using it map the underground I think is a bit of a red herring. Makes a good story, and TfL will probably be grateful for the data. But it’s not the kinda thing anyone thinks is worth developing quantum accelerometers for.

Given all the GPS jamming in Russia/Ukraine the defense world needs something not GPS based that works. The civilian world also needs this since they are often hit with the same jamming (both as collateral damage and intentional harm to the enemy)

At every station at least it should be very cheap to install optical markers that could allow for many opportunities for recalibration.

Agreed, but then said optical markers regularly positioned in tunnels and a map of the tube is enough to position yourself already. That's likely how it works right now, and it's fine.

The article says they just use wheel sensors. Most of them time they don't need to be very accurate so that is more than good enough, but in stations much higher accuracy should be needed and so they need an additional correction there. (note that I said should: I'm a strong believer in automated edge of platform doors which require stopping so the train doors align. Few systems in the world have this though)

A number of parts of the underground already have platform edge doors, and train stopping locations are tightly controlled regardless of the presence of platform edge doors.

the accumulation is due to integration when converting acceleration to distance, not the accelerometer itself

The accumulation might be from the integration, but the error is clearly from the accelerometers. Smaller error means smaller accumulated error, which means your magic box is usefully accurate for longer.

I wonder if it theoretically could (or already does) benefit from something like this: https://www.youtube.com/watch?v=nCg3aXn5F3M

It’s quantum so it’s better

Don't trains run on a fixed path, meaning we can use more traditional "positioning systems" like, umm, math? Or placing passive sensors or paint a big number on the wall?

Maths isn't a sensor technology. Wheels slip, air pressure fluctuates, rotary counters misread, errors accumulate, so better raw data could get you a more precise estimate of your current position. Various position technologies are already in use (e.g. "balise") but they have their limits if you're after cm-level accuracy.

That said, I'm not buying that quantum INS is required for this over more established techniques like visual-inertial odometry. However, the UK seems to have a whole bunch of active quantum INS research projects, so I wonder if the tube stuff is just a weak public cover for the military/civil emergency applications that make the technology actually interesting.

> so I wonder if the tube stuff is just a weak public cover for the military/civil emergency applications that make the technology actually interesting.

The project is quite explicitly a defence project. IanVisits is just a transport blog, so the article focus on the transport aspect of this project. But reporting else where makes it quite clear this a defence project.

https://www.theguardian.com/science/article/2024/jun/15/lond...

The only reasons for the Tube being involved is as a test bed for the technology. It’s cheaper to put this box on a train and test it, than it is to put it on a boat or a submarine. It’s also handy that the researchers involved are based in London, so the commute to their test bed is nice and short.

An experimental cryogenic device doesn't sound very good in terms of reliability. A train could have a few free rubber-coated wheels dedicated for precise odometry, there could be cameras with optical flow/reference markers on the tunnel walls every so often, virtually anything seems to be better than sinking millions in quantum devices.

Maybe they used trains as an example because that's the only application for which the accelerometer technique is currently sufficiently accurate.

The worlds longest linear encoder?

Cool technology, but seems a bit of an extreme effort just to check train position. Why can't they just use markers on the tunnel walls (or under the tracks)?

> Naturally, trains already have track-based location systems, but they are usually based on a train being within a “moving block”, so their accuracy is down to metres rather than centimetres. If you want to monitor track conditions, the more accurate the location of the suspected fault, the less time staff spend repairing it.

Agree in principle though, is this extreme precision really needed?

The lines with platform edge doors (Elizabeth, half of Jubilee) do need to be in the right place with 10-20cm accuracy.

Surely this has to also be a problem long solved by classical physics?

Track defects tend to be quite small, so yes, for track maintenance monitoring the extra precision is the difference between "somewhere in that rail, and you'll have to use expensive equipment to find out where before you start work" and "that's the bit that needs to be fixed."

> their accuracy is down to metres rather than centimetres. If you want to monitor track conditions, the more accurate the location of the suspected fault, the less time staff spend repairing it.

Considering the tracks are linear, I would estimate the additional time needed to locate a fault within two meters as compared to two centimeters at "negligible".

On the alternative assumption that the faults are too small for humans to detect and we just need to replace the affected track... I would also estimate the additional time needed to replace two meters of track, as compared to two centimeters, at "negligible". It doesn't actually take less time to cut out a specific 1cm strip (containing no visible indications!) from a piece of cloth as to cut out a 1m strip that includes the 1cm strip somewhere.

Do they repair track cracks? I'd expect they replace the entire track section - both rails - as long as they are there. Get within however much they can do in a work shift and good enough.

So.... how would it be faster to do that if you knew the location of the fault to within 2cm instead of knowing it to within 2m?

I don't know, that is why I'm asking.

My general thought as someone who doesn't know how rail maintenance is done is that rails should not crack in normal operation. If there is a crack that implies either the rail is end of life anyway and you replace it, or there is likely a manufacturing problem and you want to replace all the rail from that branch. Either way rail is manufactures in long sections (20 meters is my guess, but that is slightly educated guess that I won't stand by), so you would only need within a few meters to find the track section in question.

However I don't know how track maintenance is done. It is entirely possible that they grind/cut out the crack and then fill in (either cast in place, fill with weld, or just replace a a few cm) and in that case you would need to know within a few mm (though maybe inspection could find it if you are within a few cm).

Again, I am not a rail expert here. This is a place where I want to know and thus would like an expert to say. (though likely no experts are reading this...)

You might be at Victoria Station, but there's a slim but tangible chance you are in fact in Paddington.

We won't know until someone looks at you. Until then you are on every station possible in the underground sorted of smeared out into a probability wave.

... Mornington Crescent!

Quite a dumb application of quantum physics. Cooling something to near zero, which takes energy, and is complex, and big, in order to obtain precisions which are magnitudes higher than what is necessary. As if one would use CPUs to prop up an uneven table and would boast that they use microelectronics.

"Instead of relying on conventional sensors, these devices use clouds of atoms cooled to near absolute zero. At those temperatures, atoms start to behave strangely — acting as both particles and waves. As the atoms “fall” through a sensor, their wave patterns shift in response to acceleration. Using what’s effectively an ultra-precise optical ruler, the system can read these changes with extraordinary accuracy, without needing satellites at all."

Quite an ignorant comment. It’s literally nothing like using a CPU to prop up an uneven table.

It’s precisely the application of quantum physics that enables current prototypes of these IMUs to achieve 1-2 orders of magnitude less position error accumulation vs. state-of-the-art gyroscopes. Think 0.1m/min vs. 10m/min.

Obviously the tube isn’t the holy grail of applications, it’s just a test bed to improve the technology. Think about why GPS is useful. Imagine that, but entirely self contained.

See comment by "Traster".

If you have fixed beacons with a known location, couldn't the devices work out their location?

Seems like an engineered solution to rely on "quantum".

As others have suggested: this is a test for military technology. When the enemy has jammed your fixed beacons you can't work out your position from them. Probably nobody will do this on a train (and they have other technology anyway), but for an airplane this is a real problem.

The signals bounce off the tunnels a lot

1.25 millions doesn’t even buy you the laser to cool the atom gas. It’s for sure interesting, but to put this on a train, we will have to solve a few more questions. like for example, how do you operate an optical table on a bumpy metro.

Those are the important questions that the people who need this need answered. Trains have better options than this, but there are some applications that don't and trains are an easy place to test whatever you are trying, if it works on a train you can then build the real thing (which is often an airplane or missile - with power an weight limitations that don't apply to trains)

In the end, it's all a series of tubes.

A series of tubes that could contain big trucks. Tube-truck duality.

This is a weird application for such sensors. The train may be used as a test platform. DARPA launched a program to develop quantum sensors that are reliable outside the lab recently [1].

[1] https://www.nextbigfuture.com/2025/10/darpa-developing-quant...

> As long as the vehicle has a known starting point, quantum accelerometers can continue tracking its movement accurately, regardless of what’s happening above the atmosphere.

So this is a much more precise inertial guidance system replacement? If true, I'd expect UK MoD to be involved to the point of making technology a military secret, but clearly it didn't happen.

The title is more than a bit of a stretch, but interesting none the less.

Considering the popular image of quantum mechanics, i m not sure people would want to be "schrodinger's passengers". It's unclear how reliable is the system in the long term

I would have thought a simple device that counts the number of times the wheel has rotated would be vastly simpler and cheaper, and surely accurate to within a few centimeters.

It wouldn't. Wheels slip in the wet, they develop flat areas with differential friction, minor differences in wheel circumference would soon add up over long distances, and all dimensions change with temperature.

But fiberoptic gyro navigation is already a thing, and has been for decades. It's not super cheap, but it's cheap enough for an industrial application like this one. So I'm not sure what problem is being solved here.

Engaging with the substance of the suggestion ...

* What would the Central Limit Theorem give us if a significant number of wheels on loco's and carriages were instrumented and ID logged over the years.

* What drift patterns and correction factors can be observed and fine tuned by passing {many} wheel streams through station to station and line end to line end "known distance" corrections.

For half a century or so aircraft height was largely done with air pressure alone.

To this day GPS accuracy is "improved" by logging the "drift" of fixed point stations giving us filter coefficients to correct for atmospheric wobble, acute angle uncertainty, as yet uncorrected time drift, etc.

For half a century or so aircraft height was largely done with air pressure alone.

It still is. Flight levels (20000 feet and above in the US) are defined by air pressure without reference to current atmospheric conditions (i.e. the actual altitude of FL200 can vary based on atmospheric conditions). Below 19000 feet, altimeters are calibrated to local conditions which are given to the pilots by air traffic control.

Still used, sure, but not solely ( 'alone' ) - it's augmented by GPS height, feedback from ground radar, onboard radar facing down and forward, etc.

See: https://en.wikipedia.org/wiki/Altimeter that list five types (including trad. air pressure type)

If it's wet inside a tube tunnel something has usually gone very wrong surely?

Also trains have quite a large number of axles, I'm sure you could reduce error by counting all of them.

Trains in tubes have a lot of suction that draws in condensation from outside air.

A curious advantage of instrumenting all axles for wheel turn counting is calibration against actual known distances (end to end, etc) over time reveals slow wheel degradation that can trigger carriage maintainance.

Not all operators do this, it's been done at various times for heavy rail mining operators with loooong trains and heavy daily usage.

A number of the tube lines have large sections above ground.

It has to be for research purposes or something. They own the whole system of trains and tunnels, so there’s got to be loads of ways they can easily implement to see if a train passes a particular point.

How about an RFID chip (costs nothing) on every sleeper. Little database and you know exactly where you are. Accurate a few centimeters

Universal Signalling are a company doing exactly this to give better train location information to signallers.

https://www.universalsignalling.com/news/latest#h.12nuchpah7...

Or maybe a camera underneath the train? It could be the same kind of thing used in optical mice, or just count the sleepers

Not bad, but practically have you seen the underside of many trains?

The spit up of grease, leaves, metal particulate from rail and wheel grind, etc gets pretty thick.

The vision challenge is to cheaply and effectively keep a clean lens.

There's a similar challenge in monitoring production conveyor belts in mineral processing - vibration, tempreture, rocks kick about, dust make for a savage environment.

But then they couldn't use the word quantum

I would put money on it using dead reckoning and tunnel markers. Quantum technology isn't something you can hire the lowest bidder to repair or upgrade.

Trivially solvable problems held back by entrenched bureaucracy. Needs 10.000 raspberry pis not quantum.

Anyone here could solve this overnight with todays tech.

Call me sceptical, but I suspect if this works very well it will be used in weapons.

Which then leads me to think the whole thing is a smoke screen, not sure why

100%, but smoke screen is over selling it. From the article

> The project is being carried out in collaboration with Transport for London, QinetiQ, PA Consulting, Imperial College London and University of Sussex.

QinetiQ is a UK weapons developer, probably the UKs largest. So the defence angle isn’t really being hidden.

Older articles on this project from elsewhere outline the defence angle even more explicitly:

https://www.theguardian.com/science/article/2024/jun/15/lond...

But IanVisits is a transport focused blog, so the article has a transport focus, rather than a defence focus.

As to why any of this is happening on the underground, that’s pretty simple. Tube trains are a good real world test bed for this technology. Shove your quantum box on an existing train, drive it through the existing tunnels a few times as part of a normal tube service. Compare the run result and validate how accurate your technology is.

It’s a lot cheaper than putting it on a boat or a submarine. Not to mention Imperial College London is based in… London. They’re literally a five minute walk from a tube station.

I had missed the collaboration list,

being tested on trains makes complete sense

Yes this is weapons technologies. Many targets are fixed (on the earth). If you can fly by wire jamming technology is useless. This was solved for strategic weapons 40 years ago at great cost.

The real problem is weapons often have power and weight limitations that don't apply to trains. So you can build large, power hungry prototypes and test them on a train. Once you work out the bugs found there you then start the miniaturization and lower power versions for where you really want it (or save a lot of money by not doing that work for something that you know can't work anyway)

I looked into quantum inertial navigation a while ago because I wanted to understand - is it some weird quantum thing where they eliminate all accumulated error or not. As far as I could tell the answer is "not" - it's just a very very precise way of measuring acceleration.

Seems kind of hard to imagine that any accelerometer can be so precise you can use dead reckoning to the centimeter over a long time scale. Especially given how strong acceleration due to gravity is.

It's pretty clear from other articles the answer is not. Although from what I can see it claims to be about 50 times better than conventional methods. One claim is they tracked a plane to within a few hundreds meters for a 300 mile flight. Which is technically impressive but entirely useless for the stated purpose in this article.

I always wondered if they could use good LiDAR and SLAM to determine location.

This is very cool and very overkill.

Sounds like a grift with the only redeeming quality of advancing technology perhaps useful elsewhere. There's no way encoders on wheels with resets on stations/junctions is not enough for precise train positioning.

The train now approaching platforms 1 and 2 is the…

At the risk of being näive, Why Tube trains are still a thing?

I want underground mini buses, optimizing their stops to the passengers' inside. Most of we passengers go to hub stations.

Tubes are like a 100-year old tech without remarkable changes.

Capacity and dwell time.

27 (soon to be 30) trains per hour on the Jubilee line for instance, hundreds of people can get on and off each one.

I don't know why I was downvoted for just asking a question.

I don't get it. Suppose this scenario:

- Instead of big formations, replace each wagon with an electric minibus. - Instead of stopping in all stations, each minibus stops in, lets say 3 or 4. - Each passenger checks into the bus dinamically assigned to their stop. - Minibuses can surpass each other.

You have less dwell time, each ride is reduced to one third of the time.

Your capacity drops dramatically, by at least an order of magnitude if not more. The Jubilee line can do 30 trains per hour, 875 people per train is 26250 people per hour. Say an average minibus can hold 26 people, you'll literally need a thousand busses an hour to move everyone. And yes it all runs at capacity especially during rush hour.

Lets call them buses instead of minibuses and each one has the same capacity of a wagon.

Instead of 10 formations of 6 wagons you have 60 buses.

You have same capacity and one third of traveling time per ride.

Reply on news.ycombinator.com