Comment by 1718627440

Why is it that these fatal train accidents have happened multiple times in Spain, but never in Japan, even though Japan has been doing it high speed rail longer, and at 10x the passenger volume than Spain? Do they have some alien safety technology unavailable to Spain?

Has there ever been a case where two self driving cars collide with each other? I hate to be part of the peanut gallery, but as a totally naive person when it comes to railway safety research it really does seem amazing train collisions can’t be easily prevented with gps, lasers, sonar or various other trivial ways. Even the crash in question seems like it might have been easily prevented by having a second human in the loop. Maybe I and others just suspect the difficulty isn’t so much technological as it is financial.

> We can not design self-driving cars to the safety standard that is required for railways.

That is completely false. PTC does that, and we've had it for several decades. There are several self-driving cars operating in the world right now. I have never heard of a PTC system causing accidents; those occur in places where the system isn't required (like entering Grand Central).

You keep pretending that avoiding train collisions is too magical to achieve with current tech, but train traffic is easy to limit (but slightly harder to optimize), and systems exist to do that, because rail is unique amongst all forms of travel: if you drop speed to zero for any trains at risk of collision, they are safe. Planes will plummet; most cars are human-operated and you can get rear-ended, etc.

> Does your phone's GPS work in a tunnel?

My car's GPS does, using dead reckoning.

Trains can also use dead reckoning.

/me checks notes, I've heard that they move on inflexible things called "rails" that almost never change the location.

I agree that the problem here has no easy, general and automatic gps-based solution but I think it's helpful to keep the actual likely situations in mind. Usually we have the most trains/densest timetables close to major transport hubs where speeds are low anyway and driving 'on sight' is a good compromise of safety and keeping things working. Where speed really matters - outside of major hubs, straight ruralish routes - we simply don't usually have great density of trains AND tracks. At least from my travels in western Europe I don't think I've ever seen more than two tracks in the same direction on high-speed-segments. So if we keep that in mind and don't expect a general and automatic solution, a simple, gps-based fallback-system that requires all trains that are within a radius of X of each other (either in time or space) MUST coordinate, else they both shutdown/move on sight, could work reasonably well. In reality and for the involved parties (the trains that are close to each other), it's a very human-scale problem.

Trains can't change lanes without warning like a Nissan Altima on the Dan Ryan Expressway. The only way a train can move from one track to an adjacent track -- or get on the first track in the first place -- is if switches are set to allow the train onto that track. And on any modern high-speed line, those switches are centrally controlled by dispatch.

Keeping track of which parallel track a train was on automatically was a solved problem a hundred years ago, using relays and lights and hard-wired logic. Adding computers and GPS greatly increases the precision with which dispatch knows position and speed along the track, but knowing which track has not changed in the last century.

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?

Tunnels are controlled by a single block - only one train can ever be allowed access at once, period. Until the "current" occupant train is verifiably out of the tunnel, that block is forbidden to all.

Forests are irrelevant; rail right-of-way contains no trees, and is wide enough to assure clear "view" of satellites.

> Would you dare to cross on a rope over a deep valley, blind using only GPS readings of your phone?

Hang on, this is nothing at all like that - trains run on fixed tracks. The only bit of data we are interested in for collision avoidance is the linear offset along the track. Even the shittiest GPS fix of all-time would give a good-enough intersection with a fixed path to avoid crashing into a second stationary train in the open country-side (do you see any satellite line-of-sight blockers in the crash footage?)

The way I think of it, divide the tracks into segments. Before you put a train on a segment, you must ask a question - is there a train on it. There are only two answers possible, No, and something else. If it is something else, stop. Don't let the train on that segment.

There are optimizations possible like make segments smaller etc but the original algorithm is trivial. If we can't figure things out, we make segments longer and we slow down the trains until we can. I am positive for a sufficiently large track segment and a sufficiently slow train, we can always answer the question with a no.

who said anything about only using gps? I'm saying in addition to their current system!

Why would you need that many? Trains can keep pretty reasonable track of the distance between two balises using odometers.

There are also other solutions, e.g. LZB uses a phase change of the signal every 100 meters as an additional precision signal.

I guess I'm confused by why non track -based solutions are necessary? ETCS L2 would gracefully handle a system restart and automatically force a 40kph speed limit until a train traversed the unknown track proving no occupancy, and allowing proceeding trains to go faster. This all works on a track-based system.

Thank you for your time and knowledge.

PTC is a performance standard, not a concrete system. Some PTC systems do seem to use GPS for train positioning, which does seem surprising from a European train protection system perspective which all seem to completely avoid reliance on GNSS so far.

I guess a much less dense and largely overground network, mainly used by fairly slow moving freight trains as opposed to highspeed passenger connections, changes requirements somewhat.