Hacker News
points by esafak 10 hours ago | 0 comments
It's very easy since there is limited ability for trains to change tracks, so the location history of the train is a strong indicator of which track it is on. Even Google Maps can tell you which lane you are in since god knows when, and cars can freely change lanes. And that's just consumer software. For something commercial like a train they could trivially solve this problem with something more robust. But I'm not familiar with the Spanish system; how much of the engineering did they do?
[–] brookst 10 hours ago
There are switches that move trains between tracks. Now you need every switch to report, keep a history of that. Many trains also separate and combine. Trains sometimes go backwards to be switched.

It's not impossible. But handwaving GPS placement of trains at the correct location, direction, and track deserves more than a handwave as trivial.

[–] esafak 8 hours ago
All of those failure modes can be accommodated. I fail to see anything here a competent engineer couldn't address. I speculate it's not a technical problem, but an operational and cultural one. This is not rocket science.
[–] 1718627440 6 hours ago
But it must be also cheaper and more reliable than the current system, otherwise nobody is going to implement it.
[–] makethem6666688 5 hours ago
Two rtk receivers with LTE including a little battery backup would cost what, $2,000 per car tops?

Presumably switches already report their position to somewhere.

It's absolutely unacceptable for this problem exist in today's world.

[–] 1718627440 5 hours ago
> Presumably switches already report their position to somewhere.

Yeah and this somewhere already knows whether a crash is going to happen. In which case your solution brings no benefit. You can't rely on the correctness of a system, which you want to make an alternative to, to help in the case the original system fails.

This discussion focuses on a non-problem (granted partly due to an imprecise comment of mine). The system knows where all the trains are and also that a crash will happen, if the decision is made, that then indeed was made.

What is needed is that the system needs to convince the operator there is still a problem, after it proved itself unreliable. My suggestion would be a log of the constraints the prover encountered while determining whether there is a conflict. This certainly already exists somewhere in the background, but I am not aware whether that is currently shown to operators in deployed systems. It does exist somewhat in the research systems we use in university.

[–] SJC_Hacker 48 minutes ago
Multiple independent systems. Easy to ignore a single system, but two or more ? Would have to have a death wish
[–] 1718627440 31 minutes ago
When I wrote that comment I would have answered, that you already need to distrust a system, that of course still have the authority to sound like it would be right. Also from other accident reports, my understanding is that relying on yet another system causes people to make even more errors, while when they start to manually assess the situation from first principles, they are able to grasp the fundamental error, because they are questioning every assumption, as they are reconstructing a world model different from the automatic system. But the latter takes more time and effort, so people tend to do the former.

The thing is that in modern automated dispatching you only have the same world model as the automatic system, so when there is an error you have the same world model as the system, but that just detected, that that does not equal the actual state of the world. And often the error is in fact a false positive.

But now, I actually think that this accident was caused by several systems being in effect: https://news.ycombinator.com/item?id=50298796

[–] AnimalMuppet 10 hours ago
Yes, it can tell you. How perfectly, though? Perfectly enough to bet 60 lives on?

When Google Maps gets wrong what lane I'm in, nobody dies. It can be right 99% of the time, that's good enough. It's not good enough for a train system, though - at least, I suspect it's not. I'd want a lot more than "it works in my car" before I trusted it.

[–] californical 9 hours ago
Can we just have it as an additional data point, on top of what we should already know from where the train should be from switching history? If we get 99% accuracy, is that enough to check with an operator “specify which track you are in, left or right” when the measured value is different than the expected?
[–] 1718627440 4 hours ago
In addition to that being unpractical, the train doesn't even know the state of switches, i.e. not whether it is on the same track as some other position. It's just that this is undecidable for a single train. (This is what my first comment was about, it's simply not known how to solve it on the level of a single train, likely because it is impossible.) However the answer for what to do if a train can't trust the network, is don't move. Or move by sight, which is limited to 40km/h for good reason.

On the other hand this is a complete non-issue and solved for more than a century on the network level. So there is zero need to invent satellite or vision based train detection at worst even using statistical systems, when you can use deterministic systems, for example using the fact that a train is out of metal, so it's detectable by the train closing a electric circuit between the rails. Or using the sensors that are installed at every signal, switch or other positions for precisely this question.

[–] lxgr 5 hours ago
HN does railway safety... The Dunning-Kruger level in this thread is ridiculous.

As a passenger, I'm very glad software engineers haven't (fully) taken over aviation and railway safety yet.

[–] kube-system 10 hours ago
Google maps gives me the directions for the express lanes when I'm in the normal lanes and vice versa like 50% of the time, and that's an order of magnitude greater error in distance than the distance between adjacent train tracks. And several orders of magnitude greater failure rate than is acceptable for a safety system.
[–] makethem6666688 5 hours ago
RTK is more than capable of figuring out what track you are on
[–] 1718627440 5 hours ago
It can be on the other track right now, but change to your track in the future. Or in other words, the topology of the network changes in operation. Whether the track you are on and the track another train is on, is the same or not is not decidable without knowing the current network state.