4 ms·

The GPS, radio communication, etc., that the article talks about are not the important bits of positive train control IMO. The important bits are "positively"

The GPS, radio communication, etc., that the article talks about are not the important bits of positive train control IMO. The important bits are "positively" enforcing safety -- reaching a safe state even if a rail vehicle violates the instructions the signalling system provides it. America's PTC mandate was triggered by the tragic Chatsworth crash back in 2005 or so, which would have been prevented if the red signal shown to the distracted train operator had been positively enforced. 1920s technology can provide enforcement. New York City subway and the Berlin S-Bahn both use old-school wayside trip cocks that raise next to red signals, and if a train passes a red, a mechanism on the side of the train strikes the raised trip cock, triggering a mandatory emergency brake application.

Animats 9y ago
NYC Subway is really hard-core about train stops. There's a trip cock on every subway car, near the right front wheel, and it works by directly operating an air valve to the brakes. The train stop lever alongside the track is raised by a big spring and lowered electrically, and the signaling system senses when it's physically up, ready to trip. Only then can the preceding train stop lower. There are always several train stops up between trains. A signaling failure or power cut will raise the train stops. "The central principle of signal placement and engineering is that the control lengths of the signals be so designed such that when a train running at the highest possible speed under the worst weather conditions, with any semblance of functional brakes, is tripped, it will come to a halt before encountering the other train or obstruction that is causing the signal to be red." Some signals have timers, for speed control. Go through a block too fast, and the signal ahead won't clear. Then the train stop will stop the train. This is common in downhill sections and ahead of curves. Resetting a trip cock in NYC requires getting out of the cab and climbing down to the tracks. This is Not Fun. Chicago has self-resetting trip cocks, which resulted in an accident in 2013.[1] A train parked in a service yard had a water leak into the control cables, which caused the train to move, slowly. The train hit a train stop, the brakes were applied, and the train stopped. But the trip cock reset, and the train started again. This happened five times at five different train stops. Workers noticed, but were not able to stop the train. This continued until the train hit an occupied but stopped train just outside the yard. This is why you want dumb train control. [1] https://www.ntsb.gov/investigations/AccidentReports/Reports/RAB1502.pdf https://www.ntsb.gov/investigations/AccidentReports/Reports/...
- notmtaemployee 9y ago
  For what its worth you can run a red signal if you go slow enough over the trip, however doing this without approval from rcc will get you suspended or more
- mrbabbage 9y ago
  WOW! That Chicago story is straight up bizarre. Thanks for sharing that. I'm not sure, though, that dumb train control is necessarily the answer. The Chicago system looks pretty dumb (though I know nothing about it), certainly no more complex than the New York system, but the fact that the CTA system self-reset seems to imply it cannot be safe without a human in the cab. This misfeature is a problem regardless how dumb or complex the signal system is. Also, my hometown has one of the world's first automatic train operation systems (BART), and to my knowledge it hasn't had a revenue service malfunction leading to a crash since October 2 1972, less than a month after it opened. However, the train control system did lead to a good deal of litigation and project timeline overruns. I think the take-home lesson is that brand new technology is extraordinarily painful to debug and deploy; it's always going to be better to re-use train control that someone else debugged on their railway rather than re-inventing the wheel for your project.
  - Symbiote 9y ago
    Many other countries have debugged the ERTMS system (European Rail Traffic Management System, or ETCS, European Train Control System). I hope the US doen't get stuck with inventing something itself. It would certainly cost more. 20 or so non-European countries have started to use this system[1], including some very large ones. [1] http://www.ertms.net/?page_id=55 http://www.ertms.net/?page_id=55

Animats 9y ago

NYC Subway is really hard-core about train stops. There's a trip cock on every subway car, near the right front wheel, and it works by directly operating an air valve to the brakes. The train stop lever alongside the track is raised by a big spring and lowered electrically, and the signaling system senses when it's physically up, ready to trip. Only then can the preceding train stop lower. There are always several train stops up between trains. A signaling failure or power cut will raise the train stops. "The central principle of signal placement and engineering is that the control lengths of the signals be so designed such that when a train running at the highest possible speed under the worst weather conditions, with any semblance of functional brakes, is tripped, it will come to a halt before encountering the other train or obstruction that is causing the signal to be red." Some signals have timers, for speed control. Go through a block too fast, and the signal ahead won't clear. Then the train stop will stop the train. This is common in downhill sections and ahead of curves. Resetting a trip cock in NYC requires getting out of the cab and climbing down to the tracks. This is Not Fun. Chicago has self-resetting trip cocks, which resulted in an accident in 2013.[1] A train parked in a service yard had a water leak into the control cables, which caused the train to move, slowly. The train hit a train stop, the brakes were applied, and the train stopped. But the trip cock reset, and the train started again. This happened five times at five different train stops. Workers noticed, but were not able to stop the train. This continued until the train hit an occupied but stopped train just outside the yard. This is why you want dumb train control. [1] https://www.ntsb.gov/investigations/AccidentReports/Reports/RAB1502.pdf https://www.ntsb.gov/investigations/AccidentReports/Reports/...

notmtaemployee 9y ago
For what its worth you can run a red signal if you go slow enough over the trip, however doing this without approval from rcc will get you suspended or more
mrbabbage 9y ago
WOW! That Chicago story is straight up bizarre. Thanks for sharing that. I'm not sure, though, that dumb train control is necessarily the answer. The Chicago system looks pretty dumb (though I know nothing about it), certainly no more complex than the New York system, but the fact that the CTA system self-reset seems to imply it cannot be safe without a human in the cab. This misfeature is a problem regardless how dumb or complex the signal system is. Also, my hometown has one of the world's first automatic train operation systems (BART), and to my knowledge it hasn't had a revenue service malfunction leading to a crash since October 2 1972, less than a month after it opened. However, the train control system did lead to a good deal of litigation and project timeline overruns. I think the take-home lesson is that brand new technology is extraordinarily painful to debug and deploy; it's always going to be better to re-use train control that someone else debugged on their railway rather than re-inventing the wheel for your project.
- Symbiote 9y ago
  Many other countries have debugged the ERTMS system (European Rail Traffic Management System, or ETCS, European Train Control System). I hope the US doen't get stuck with inventing something itself. It would certainly cost more. 20 or so non-European countries have started to use this system[1], including some very large ones. [1] http://www.ertms.net/?page_id=55 http://www.ertms.net/?page_id=55