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My 1955 Citroen (designed in 1933) has the original speedometer and speed sender cable. It's accurate (GPS tracked) to within 2-3km/h. This is with nearly 70k miles on the clock. This is the same mechanism Citroen used in their 2CV from 1946 until 1992. Most Model A Fords from 1929 have their original speedometer (which works fine, might need greasing every few decades though). With tens of thousands of miles on them.

We are way, WAY beyond "hardware engineering is hard", this is "this was a solved problem a century ago, using archaic means". I am happy to hand-wave away all sorts of problems but speedometers were 100% a solved problem many many years ago and no allowances or leeway should be given for this specific problem. Zero.

GPS doesn’t work underground though.

But the problem described in the article seems rather unique to NYC and one has to ask how other subway systems manage just fine without artificial slowdowns.

I'm pretty sure the person you're replying to was saying that the original speedometer mechanism in their 1955 car is accurate to within 2-3 km/h of the speed reported by GPS, and thus suggesting that this pre-GPS mechanism ought suffice for the subway / be better than whatever they currently use.

You can simulate the GPS if you wanted to, drop cables down to pipe through the real signal, or simply do very basic positioning with custom radios underground.

OP is correct. We have significantly better techniques.

For what it's worth, a metro train does around 100,000 miles per year.

And every single one of them invariably is when i'm not onboard.

So do most normal trains, and they all have speed gauges that work fine, rain snow or dust. Clearly the technology exists.

NYCT was, until this year, operating R32 trainsets still limping along from the 1960s that were repeatedly life-extended as newer (1970s) train types experienced structural failures & delivery of their planned replacement trains was repeatedly delayed. It's obviously possible to maintain old trains, but generally expensive & challenging, especially when they're still doing 100,000 miles a year, and things clearly slipped.

Especially because the trains were operating well past their planned lifespan because their replacements were ordered but not delivered, so the major overhaul that would that would normally be done to life-extend a train kept running for decades didn't happen.

That's fair. And also, the environment is probably different, as in more dust in the tunnels, etc.

But to stick with French technology, the Paris metro, for all its issues, does have working speed gauges. Some lines still use rolling stock from the 60s and 70s.

So, considerably less than the average articulated lorry, or indeed Skoda Octavia?

No, very much around the average for an articulated lorry.

But metro trains stop and start every 2-5 minutes all day, last 40+ years, but also travel on a very smooth "road".

You must live somewhere without a lot of road haulage, or without any taxis.

I don't have the link to hand. It was figures from a forum for drivers discussing what "average mileage" would mean on a British articulated lorry.

Go fix it then :p

They're slowly switching each line to a CBCT based signaling, the lines that do have it are vastly improved. Aside from all the budget and construction corruption issues of the MTA, it's difficult to do because the NYC subway is expected to run 24/7. And so upgrades need to be scheduled around limited times during which lines can be shut down, usually just a few hours overnight. And also keep the old system running while upgrades happen which tend to take years for a full line upgrade.

https://en.m.wikipedia.org/wiki/Communications-based_train_c...

Anecdotally, CBTC is a greeat improvement in many scenarios. On long and straight stretches between express stops, trains now fly along (probably at least 40mph; never got to see inside the TO’s cab). And upon approaching an occupied station, they slow down to a crawl but keep moving until about 25-50 feet before the station, instead of being stuck several fixed blocks back before the station. Thus there is much greater track capacity = more (potential) trains per hour, and generally more reliable trips.

Unfortunately, it's a pretty tired HN comment to posit something as being more simple than it actually is.

Even during the pandemic there were widespread complaints about the signal work on the L line that has now transformed it from one of the worst lines to one of the best. There's no such thing as "simple work" on a system that millions of people depend on for consistent uptime.

If you count in the financial constrait that service is working under, sure. But meassuring the speed/position of a rail vehicle on a piece of track reliably is not an unsolvable feat of hardware engineering. It is done elsewhere and it is done elsewhere where similar constraints for uptime exist.

Theoretically by that logic we could argue that trains are hard because making the motors for them is non-trivial. It is non-trivial, and depending on your standards it might be even hard. But it is essentially a solved problem. You want a motor? You get one from the big companies, let them design one or use one from an existing similar train. Same thing goes for measuring speed. You want it? Create a team researching which ones to get.

Like in many places NY infrastructure has it's best days long behind itself and it is a wonder it still works. That infrastructure is in dire need of modernization and it has been for a while. The reason this is not done is not because it is hard or impossible to do. It is just expensive.

You’re correct, it’s not unsolvable, which is why it is being solved. It’s just a slow process, for the reasons mentioned.

Don't forget WET environment. Due to the closeness to the surface and the era most of the tunneling was done, there's not just high humidity but actual water in the tunnels and stations quite often.

Is there a term for using multiple redundancy and statistics to solve issues, for example using 5x 99% reliable sensors rather than a single 99.99% sensor that costs 100x more?

"redundant systems"

engineering

> I assume it's a budget limitation driving slow progress.

The budget is enormous, it's just being embezzled from top to bottom.

So.. what tech do normal, above ground trains use?