8 ms·

> Drivers go even slower than 45mph because the train is automatically stopped when going faster (as you would want to do if there was a train ahead) instead of

> Drivers go even slower than 45mph because the train is automatically stopped when going faster (as you would want to do if there was a train ahead) instead of just giving a warning and letting the train keep going. Is there any protection system that allows the maximum speed to be exceeded with just a warning? Certainly all those I can think of (TPWS, TVM, PZB, or something more modern like ETCS) have a given maximum speed you're allowed to exceed by a small tolerance but nothing more (though, of course, the system may be isolated, but that's not allowed in passenger service). The failing here sounds like it is setting the line speed to 45mph rather than anything higher.

thunfischbrot 9y ago
That, and considering a speed limiter instead of stopping the train when traveling too fast. What good does it do to stop the train when going too fast instead of limiting it in the first place?
- gsnedders 9y ago
  The general principle in railway movement authority is that there's an easy fail safe: stop moving. If nothing moves, there can be no collision. If something goes wrong? Stop the train. You can then figure out what went wrong and address it with no risk. Things shouldn't be going wrong, speeds shouldn't be being exceeded, etc.
  - Retric 9y ago
    Even then you should have some give aka Speed of X+3 MPH break, speed of x+1 MPH warning. You want fault tolerance not simply a hard limit without feedback.
    - abakker 9y ago
      Just a PID speed controller, maybe. Then, some boundary condition where when the controllers inputs mismatched from predicted outputs, then throw an error. But, if you're trying to drive 45, and you achieve 46 with a tail wind it doesn't seem necessary or safer to stop the train.
  - ovi256 9y ago
    >If nothing moves, there can be no collision. If no train on the line moves, sure. But stoping just the one train won't improve safety, and was historically the cause of many accidents. I remember just the one in Shanghai in 201x. One train stopped, the following train hit it.
    - taneq 9y ago
      If stopping doesn't fix things, you're not using enough of it. Time for networked emergency brakes on all trains!
      - Symbiote 9y ago
        Europe may has that, with the GSM-R system. I remember reading of at least one minor accident that stayed minor because the driver to discover the landslide over the track at low speed passed the button, stopping the incoming faster train on the other track. http://www.szdc.cz/en/pro-media/tiskove-zpravy/funkce-gsm-r-stop.html http://www.szdc.cz/en/pro-media/tiskove-zpravy/funkce-gsm-r-...
        gsnedders 9y ago
        AFAIK, this is just a Czech extension to GSM-R. That said, I recall various instances of GSM-R being used in the UK to broadcast a general message to stop, albeit one that the driver has to act upon.
    - tjohns 9y ago
      That shouldn't happen under normal circumstances. That's whole idea behind railway signals. Rail segments (blocks) are supposed to be automatically locked off while a train is on that segment of track. No two trains can be on the same block at the same time. If a train stops, the signal guarding that block will remain red, ordering the next train to stop before it gets anywhere close.
    - 7dare 9y ago
      In fact, the crash at the origin of this whol debacle was caused by a train being stopped on the tracks and being rear-ended by another.
- Zak 9y ago
  I think the idea is that if the operator is incapacitated or the control system is malfunctioning, stopping the train is the safest solution. While true, a multi-shot timer would be equally effective in detecting such a situation.
LolWolf 9y ago
There are... one of them is given in the article—which calls the current system "one-shot"—there exists k-shot variants which allow for at least k opportunities/warnings before automagically braking. The (apparent) reasoning behind the MTA not putting these (for some number k>1) was "safety," but the article states that, in general, the timing circuits are much more complicated therefore resulting in a higher cost.[0] Additionally, a simple case would be to have circuits which do, after a certain point in time, force slow (but safe) braking into the required velocity---I could see problems with robustness here due to the specific application, but at least in theory this should be possible (we all know the difference between theory and practice, though). On the general case, this all seems a little weird to me since it seems like such a trivial problem (increase error bands, etc), but may have some deeper reasons that aren't explained in the article(?). It's quite difficult to tell since the primary sources are not provided at any point in time. --- [0] I'm reluctant to accept this claim since most of these things aren't done in an analog way, anymore. Timing is a matter of figuring out empirical constants and programming them in directly, which, while not totally trivial, seems to have roughly the same difficulty (amortized over all of the lights that need to be put in) as doing the timing for a 'one-shot' light.
- azernik 9y ago
  Indeed, this is my understanding of how most non-US high-speed-rail controllers work - the driver sets a target speed, the physical location on the track has a maximum speed, and the traffic system looks at the cars ahead and sets its own maximum speed (e.g. zero when another train is right in front of you). Instead of stopping the train when the driver exceeds the speed limit, it just uses the minimum of all the target speeds.
  - gsnedders 9y ago
    TVM is the system I'm most familiar with when it comes to HSR (used on the LGV in France and HS1 in the UK); it has a variable overspeed allowance (5 to 15 km/h depending on current maximum speed). If you exceed that, an emergency brake application occurs.

thunfischbrot 9y ago

That, and considering a speed limiter instead of stopping the train when traveling too fast. What good does it do to stop the train when going too fast instead of limiting it in the first place?

gsnedders 9y ago
The general principle in railway movement authority is that there's an easy fail safe: stop moving. If nothing moves, there can be no collision. If something goes wrong? Stop the train. You can then figure out what went wrong and address it with no risk. Things shouldn't be going wrong, speeds shouldn't be being exceeded, etc.
- Retric 9y ago
  Even then you should have some give aka Speed of X+3 MPH break, speed of x+1 MPH warning. You want fault tolerance not simply a hard limit without feedback.
  - abakker 9y ago
    Just a PID speed controller, maybe. Then, some boundary condition where when the controllers inputs mismatched from predicted outputs, then throw an error. But, if you're trying to drive 45, and you achieve 46 with a tail wind it doesn't seem necessary or safer to stop the train.
- ovi256 9y ago
  >If nothing moves, there can be no collision. If no train on the line moves, sure. But stoping just the one train won't improve safety, and was historically the cause of many accidents. I remember just the one in Shanghai in 201x. One train stopped, the following train hit it.
  - taneq 9y ago
    If stopping doesn't fix things, you're not using enough of it. Time for networked emergency brakes on all trains!
    - Symbiote 9y ago
      Europe may has that, with the GSM-R system. I remember reading of at least one minor accident that stayed minor because the driver to discover the landslide over the track at low speed passed the button, stopping the incoming faster train on the other track. http://www.szdc.cz/en/pro-media/tiskove-zpravy/funkce-gsm-r-stop.html http://www.szdc.cz/en/pro-media/tiskove-zpravy/funkce-gsm-r-...
      - gsnedders 9y ago
        AFAIK, this is just a Czech extension to GSM-R. That said, I recall various instances of GSM-R being used in the UK to broadcast a general message to stop, albeit one that the driver has to act upon.
  - tjohns 9y ago
    That shouldn't happen under normal circumstances. That's whole idea behind railway signals. Rail segments (blocks) are supposed to be automatically locked off while a train is on that segment of track. No two trains can be on the same block at the same time. If a train stops, the signal guarding that block will remain red, ordering the next train to stop before it gets anywhere close.
  - 7dare 9y ago
    In fact, the crash at the origin of this whol debacle was caused by a train being stopped on the tracks and being rear-ended by another.
Zak 9y ago
I think the idea is that if the operator is incapacitated or the control system is malfunctioning, stopping the train is the safest solution. While true, a multi-shot timer would be equally effective in detecting such a situation.

LolWolf 9y ago

There are... one of them is given in the article—which calls the current system "one-shot"—there exists k-shot variants which allow for at least k opportunities/warnings before automagically braking. The (apparent) reasoning behind the MTA not putting these (for some number k>1) was "safety," but the article states that, in general, the timing circuits are much more complicated therefore resulting in a higher cost.[0] Additionally, a simple case would be to have circuits which do, after a certain point in time, force slow (but safe) braking into the required velocity---I could see problems with robustness here due to the specific application, but at least in theory this should be possible (we all know the difference between theory and practice, though). On the general case, this all seems a little weird to me since it seems like such a trivial problem (increase error bands, etc), but may have some deeper reasons that aren't explained in the article(?). It's quite difficult to tell since the primary sources are not provided at any point in time. --- [0] I'm reluctant to accept this claim since most of these things aren't done in an analog way, anymore. Timing is a matter of figuring out empirical constants and programming them in directly, which, while not totally trivial, seems to have roughly the same difficulty (amortized over all of the lights that need to be put in) as doing the timing for a 'one-shot' light.

azernik 9y ago
Indeed, this is my understanding of how most non-US high-speed-rail controllers work - the driver sets a target speed, the physical location on the track has a maximum speed, and the traffic system looks at the cars ahead and sets its own maximum speed (e.g. zero when another train is right in front of you). Instead of stopping the train when the driver exceeds the speed limit, it just uses the minimum of all the target speeds.
- gsnedders 9y ago
  TVM is the system I'm most familiar with when it comes to HSR (used on the LGV in France and HS1 in the UK); it has a variable overspeed allowance (5 to 15 km/h depending on current maximum speed). If you exceed that, an emergency brake application occurs.