A new way of keeping trains apart uses magnetic signals in the tracks

But in any case, the statement you quotes smells like very sloppy statistics. There's a huge difference between saying that the pattern of permeability is unpredictable, and saying it's random -- the latter is a very strong assumption. It's easy to imagine ways the measurement could have local or global biases towards particular kinds of patterns, and the existence of any such effects would make the "one in a trillion" number completely meaningless.

On top of that, the article says magnetic permeability isn't part of the official material specifications. What happens if the railroad operator switches to a new supplier, whose process results in rails that are much more uniform?

This seems to be the research group's web page: https://www.mrt.kit.edu/disensor/ I looked for a published paper that would go into more detail, but I couldn't find one.

Here restricting the search to a radius of known starting positions should suffice.

This does have potential advantages in maybe finding track defects if set up correctly to do so? But that sort of system already exists and is installed on rolling stock, so once again it’s having to overcome the first mover advantage.

Then, in the US, they sometimes use "rail grinders" - specialized trains that literally grind the surface of the rail, in order to remove things like spalling that come from heavy use. That probably changes the magnetic signature, too, a lot faster than regular wear does.

That's not insoluble - re-map after you grind. But it does complicate things somewhat.

It’s complex because edge cases like this exist for simpler systems.

There are parallel tracks within metres of one another. Switching yards have tonnes of them and require precise knowledge of what’s in what.

GPS isn’t SIL.

Trains operate in cluttered environments with reflection and echo abound. That causes drift above the separation of tracks.

There’s way more than “a few” dead spots, and what happens if those repeaters fall over?

There’s tonnes of reasons why high tech isn’t used in train safety-critical systems. The fact you suggest an arduino as an appropriate platform for this is frankly absurd and underscores your misunderstanding of the whole issue.

Close enough isn’t good enough. Crashing doesn’t just mean you reboot or reload the program and try again. It means people die.

Edit to add: A good rule of thumb I’ve learnt after 15 years in the game: if you look at the solution to an engineering problem and then start a sentence with “why don’t they just…”, you probably don’t understand the problem well enough.

None of the things you have described are "easy problems". In fact, they are pretty difficult! Systems need to be robust, interoperable, reliable, provably correct, fail safely, deal with surprising edge cases, have long service lives, and any number of other challenges.

In any case, all you've really done here is re-invent technology that already exists. In ETCS level 2, for example, the signalling system uses known-location transmitters on the track as references and the train calculates its position in-between using on-board sensors like axle encoders, IMUs etc.

This is just a short article about an interesting new technology for providing the "train position" component of the system by building a magnetic map of the rails. Pretty cool idea – if it's accurate enough, it provides a more robust source of absolute positioning information, which is key to allowing the rollout of realising "moving-block" approaches in which trains' movement authorities are updated in real-time, allowing higher traffic density.

And it's not like nobody's thought about GPS/GNSS either – there have been extensive projects looking into the feasibility of those systems. The Russian ABTC-M system uses GLONASS, and I'm sure some implementations of PTC in the US use GPS. But again, these are difficult challenges incorporating lots of different technology and you don't solve them overnight.

If anything, they are understated. GPS is so easy to spoof that it should not be used in a vital system.

Most train control systems have two completely separate systems. One keeps trains from running into each other, and the other makes the railroad run efficiently. The first usually depends on brutally simple and fail-safe wayside equipment. The second is based on direction from a control center somewhere. The second system is capable of keeping trains separated on its own, and it's so tempting to get rid of all that wayside safety equipment, with all that gear in trackside boxes needing maintenance visits.

Track spacing is often much less than the dilution of position, which is less than ideal. This means that the area that the GPS thinks the train is, is bigger than the track width. Trains in the UK are often in cuttings, urban canyons etc, which means that the error margin could be up to 100m without much effort.

> instrumenting each switch to understand when a train switches tracks seems like an easy problem.

Only if you ignore that those sensors are now life critical, so is the connection, and the software that joins it all up.

> Tunnels? First there aren't "that" many and there aren't many long enough that a single long train shouldn't count it as occupied in the interest of safety.

again see DoP. But safety distance is related to speed. the slower the trains the shorter the distance, which means you are more likley to bump into GPS DoP issues. Also tunnels normally have cuttings, which again limit the number of satellites visible.

> It also shouldn't be a high effort to add a satellite or wireless repeater with a known location at at least each end of a tunnel.

again see safety critical equipment, you'll need fail safes

> The accelerometer can even early report rough tracks or switches.

Whats the percentage drift per second of those sensors? How often is the calibration frequency? how much vibration can it handle before it becomes inaccurate? whats the mean time to failure?

As a point, the "new" thameslink trains refused to open its doors at the thameslink station, because the station was in a tunnel, and the GPS wasn't available. This meant that the train said it didn't know where it was, so no doors open.

As a general rule of thumb, life critical stuff needs to be simple, well tested and well characterised. Failure of a system might cause a death, subtle failure is even more dangerous. This means expense.