5 ms·

> I'll confess I still don't understand why it's so hard to keep track of the position of subway cars in real time. I can think of any number of ways to do that

> I'll confess I still don't understand why it's so hard to keep track of the position of subway cars in real time. I can think of any number of ways to do that more or less trivially. That’s the source of a lot of error in project estimates, since it is trivial to come up with plans before considering the details. Phrasing it as a software project: “I don’t understand why it is so hard to find information on the internet. All of the pages are available through HTTP web servers, aren’t they? What’s wrong with just downloading them all?”

CamperBob2 8y ago
I don't follow you. How is keeping track of the position of a small number of subway cars in a limited geographical area anywhere near as abstract a problem as "finding information on the Internet?" This is a relatively-trivial engineering exercise with any number of safe, efficient solutions. If someone says it will take a billion dollars to solve the problem, that is the person you should demand proof from.
- jcranmer 8y ago
  If you need to replace physical infrastructure, well, that's where the problem is. MTA has about 6500 subway cars. Replacing all of those would tie up somewhere around 10% of the world's total yearly subway car manufacturing capacity. Okay, so maybe you only design a module that can be retrofitted onto existing cars--there's only 16 car designs on the MTA according to Wikipedia. You just have to install them on every car, which means figuring out how to slot the install times into the maintenance schedules of cars, and if your schedule slips for delivering the parts to install, well, that's going to push stuff back a few months. The problem isn't that the algorithm is hard. The problem is that the hardware didn't exist, and retrofitting hardware is a much more expensive endeavor than retrofitting software.
  - CamperBob2 8y ago
    Okay, so maybe you only design a module that can be retrofitted onto existing cars... The cars have to receive power from someplace. Wherever they are being energized, there is by definition a complete circuit to all cars being powered on that line. Likewise, wherever the cars are, there is going to be a large and reasonably well-defined impedance discontinuity on the power rail. So, as a first attempt, I'd look at some sort of reflectometry scheme. Wind a few turns of wire around the cable that feeds the third rail, inject a pulse train with Gold codes or something similar that lends itself to autocorrelation methods, and listen for echoes. This requires a grand total of $0 in hardware to be added to the cars. If it's not enough -- e.g., if the points raised in Anechoic's post render it impractical to rely on passive reflections -- then it should be possible to add a small amount of hardware at the cars to act as an active transponder, injecting its own pulse train in response to carrier-current signals from the distribution point. So now we need to connect two wires at each car, and mount a small box with some duct tape^W^W milspec fasteners. Not exactly rocket surgery.
    - inferiorhuman 8y ago
      > The cars have to receive power from someplace. Wherever they are being energized, there is by definition a complete circuit to all cars being powered on that line. Most, but not all of the time. There are dead segments in pretty much any system be it overhead or third rail. > Likewise, wherever the cars are, there is going to be a large and reasonably well-defined impedance discontinuity on the power rail. Nope. See Bill Wattenburg's demonstration with BART. > Wind a few turns of wire around the cable that feeds the third rail, inject a pulse train with Gold codes or something similar that lends itself to autocorrelation methods, and listen for echoes. What if you're not using a third rail? Basically you seem to have solved the problem under ideal circumstances but not accounted for the myriad of failure modes one will encounter in the real world. Edit: the other part you're missing is that the first C in CBTC is communication. Locating trains has been fairly well solved (inductive loops, axle counters, etc), communication as well is pretty well solved (802.11 in some cases). The secret sauce is in making everything reliable (think about it, the difference between 3 and 5 nines is a ton of pissed off commutere) AND in making sure everything behaves safely and in a predictable manner. In more traditional automatic train control setups you divide the track into fixed segments (blocks). One train can occupy a block at a time. Depending on the size of the blocks you may have to keep a block or two of separation between trains. CBTC typically implies what's called moving block. Basically the train is the block. So now you've gotta keep track of all the trains, how fast they're going, how fast they're capable of braking, etc, etc. in order to determine safe spacing and speed limits. It's a bit like automated driving. The algorithms are the secret sauce, and bugs can be fatal. SF's Muni famously had some trains go down the wrong track in the wrong direction during first stages of their CBTC deployment.
      - CamperBob2 8y ago
        Most, but not all of the time. There are dead segments in pretty much any system be it overhead or third rail. In which case you can assume the cars on those segments are pretty much right where you left them, no? I'm assuming the name of the game is to optimize speed and spacing of cars on active segments. Or do you mean the cars are basically coasting between segments under flywheel or battery power? Seems like dead reckoning is all that's needed to fill in those gaps. Maybe with a basic anticollision radar as a failsafe. :) Nope. See Bill Wattenburg's demonstration with BART I'm not familiar with him, and Google isn't helping much. Any good references? It's a bit like automated driving. Except there's no steering wheel, no pedestrians or cyclists, no weather or visibility problems, no uncontrolled vehicle traffic, and every bit of infrastructure is under your control at all times. Hence my use of 'trivial' to describe the problem. Compared to what the folks at Waymo or Cruise have to deal with, the MTA and its contractors have no excuses whatsoever.
- throwawaymath 8y ago
  > This is a relatively-trivial engineering exercise with any number of safe, efficient solutions. Your comments here are another great example of how ironic it is to hear the word "trivial" used to describe solutions. Don't you think it's a little unrealistic that your "relatively-trivial engineering exercise" suggestions are capable of successfully tackling a problem so many parties have thought about? Have you considered that your insight is actually failing to account for a variety of unknown unknowns you might have about the NYC subway system? Look at it this way: why do you suppose your suggestions haven't been implemented yet? Do you think the vast array of parties with skin in the game haven't thought of them (and are thus incompetent)? Do you think they simply aren't motivated to implement them? I wouldn't be surprised if your proposals are technically sound (especially for greenfield rail construction). But I would be absolutely shocked if the problem is anywhere near as trivial as you're making it out to be.
  - CamperBob2 8y ago
    Have you considered that your insight is actually failing to account for a variety of unknown unknowns you might have about the NYC subway system? No. Whatever the unknown unknowns are, they're not going to be that hard to work around. Keeping track of a few train cars isn't exactly the Manhattan Project, even if it has to be done underneath Manhattan. The video suggests that the total budget for the CBTC retrofit was in the vicinity of $1 billion. I hope that I either misinterpreted it, or that the budget includes other improvements, because I doubt it cost that much (in 2018 dollars) to build the initial subway routes in the first place. Look at it this way: why do you suppose your suggestions haven't been implemented yet? Do you think the vast array of parties with skin in the game haven't thought of them (and are thus incompetent)? Do you think they simply aren't motivated to implement them? I think the vast array of parties with skin in the game have treated said "skin" as a target for optimization in itself. I'll readily admit they're likely better at that game than I am.
    - throwawaymath 8y ago
      How can you say with any confidence that the unknown unknowns you have won’t be very hard to work around? That’s entirely the point of unknown unknowns - you don’t know what you don’t know.
- deleted 8y ago
  [deleted]

CamperBob2 8y ago

I don't follow you. How is keeping track of the position of a small number of subway cars in a limited geographical area anywhere near as abstract a problem as "finding information on the Internet?" This is a relatively-trivial engineering exercise with any number of safe, efficient solutions. If someone says it will take a billion dollars to solve the problem, that is the person you should demand proof from.

jcranmer 8y ago
If you need to replace physical infrastructure, well, that's where the problem is. MTA has about 6500 subway cars. Replacing all of those would tie up somewhere around 10% of the world's total yearly subway car manufacturing capacity. Okay, so maybe you only design a module that can be retrofitted onto existing cars--there's only 16 car designs on the MTA according to Wikipedia. You just have to install them on every car, which means figuring out how to slot the install times into the maintenance schedules of cars, and if your schedule slips for delivering the parts to install, well, that's going to push stuff back a few months. The problem isn't that the algorithm is hard. The problem is that the hardware didn't exist, and retrofitting hardware is a much more expensive endeavor than retrofitting software.
- CamperBob2 8y ago
  Okay, so maybe you only design a module that can be retrofitted onto existing cars... The cars have to receive power from someplace. Wherever they are being energized, there is by definition a complete circuit to all cars being powered on that line. Likewise, wherever the cars are, there is going to be a large and reasonably well-defined impedance discontinuity on the power rail. So, as a first attempt, I'd look at some sort of reflectometry scheme. Wind a few turns of wire around the cable that feeds the third rail, inject a pulse train with Gold codes or something similar that lends itself to autocorrelation methods, and listen for echoes. This requires a grand total of $0 in hardware to be added to the cars. If it's not enough -- e.g., if the points raised in Anechoic's post render it impractical to rely on passive reflections -- then it should be possible to add a small amount of hardware at the cars to act as an active transponder, injecting its own pulse train in response to carrier-current signals from the distribution point. So now we need to connect two wires at each car, and mount a small box with some duct tape^W^W milspec fasteners. Not exactly rocket surgery.
  - inferiorhuman 8y ago
    > The cars have to receive power from someplace. Wherever they are being energized, there is by definition a complete circuit to all cars being powered on that line. Most, but not all of the time. There are dead segments in pretty much any system be it overhead or third rail. > Likewise, wherever the cars are, there is going to be a large and reasonably well-defined impedance discontinuity on the power rail. Nope. See Bill Wattenburg's demonstration with BART. > Wind a few turns of wire around the cable that feeds the third rail, inject a pulse train with Gold codes or something similar that lends itself to autocorrelation methods, and listen for echoes. What if you're not using a third rail? Basically you seem to have solved the problem under ideal circumstances but not accounted for the myriad of failure modes one will encounter in the real world. Edit: the other part you're missing is that the first C in CBTC is communication. Locating trains has been fairly well solved (inductive loops, axle counters, etc), communication as well is pretty well solved (802.11 in some cases). The secret sauce is in making everything reliable (think about it, the difference between 3 and 5 nines is a ton of pissed off commutere) AND in making sure everything behaves safely and in a predictable manner. In more traditional automatic train control setups you divide the track into fixed segments (blocks). One train can occupy a block at a time. Depending on the size of the blocks you may have to keep a block or two of separation between trains. CBTC typically implies what's called moving block. Basically the train is the block. So now you've gotta keep track of all the trains, how fast they're going, how fast they're capable of braking, etc, etc. in order to determine safe spacing and speed limits. It's a bit like automated driving. The algorithms are the secret sauce, and bugs can be fatal. SF's Muni famously had some trains go down the wrong track in the wrong direction during first stages of their CBTC deployment.
    - CamperBob2 8y ago
      Most, but not all of the time. There are dead segments in pretty much any system be it overhead or third rail. In which case you can assume the cars on those segments are pretty much right where you left them, no? I'm assuming the name of the game is to optimize speed and spacing of cars on active segments. Or do you mean the cars are basically coasting between segments under flywheel or battery power? Seems like dead reckoning is all that's needed to fill in those gaps. Maybe with a basic anticollision radar as a failsafe. :) Nope. See Bill Wattenburg's demonstration with BART I'm not familiar with him, and Google isn't helping much. Any good references? It's a bit like automated driving. Except there's no steering wheel, no pedestrians or cyclists, no weather or visibility problems, no uncontrolled vehicle traffic, and every bit of infrastructure is under your control at all times. Hence my use of 'trivial' to describe the problem. Compared to what the folks at Waymo or Cruise have to deal with, the MTA and its contractors have no excuses whatsoever.
throwawaymath 8y ago
> This is a relatively-trivial engineering exercise with any number of safe, efficient solutions. Your comments here are another great example of how ironic it is to hear the word "trivial" used to describe solutions. Don't you think it's a little unrealistic that your "relatively-trivial engineering exercise" suggestions are capable of successfully tackling a problem so many parties have thought about? Have you considered that your insight is actually failing to account for a variety of unknown unknowns you might have about the NYC subway system? Look at it this way: why do you suppose your suggestions haven't been implemented yet? Do you think the vast array of parties with skin in the game haven't thought of them (and are thus incompetent)? Do you think they simply aren't motivated to implement them? I wouldn't be surprised if your proposals are technically sound (especially for greenfield rail construction). But I would be absolutely shocked if the problem is anywhere near as trivial as you're making it out to be.
- CamperBob2 8y ago
  Have you considered that your insight is actually failing to account for a variety of unknown unknowns you might have about the NYC subway system? No. Whatever the unknown unknowns are, they're not going to be that hard to work around. Keeping track of a few train cars isn't exactly the Manhattan Project, even if it has to be done underneath Manhattan. The video suggests that the total budget for the CBTC retrofit was in the vicinity of $1 billion. I hope that I either misinterpreted it, or that the budget includes other improvements, because I doubt it cost that much (in 2018 dollars) to build the initial subway routes in the first place. Look at it this way: why do you suppose your suggestions haven't been implemented yet? Do you think the vast array of parties with skin in the game haven't thought of them (and are thus incompetent)? Do you think they simply aren't motivated to implement them? I think the vast array of parties with skin in the game have treated said "skin" as a target for optimization in itself. I'll readily admit they're likely better at that game than I am.
  - throwawaymath 8y ago
    How can you say with any confidence that the unknown unknowns you have won’t be very hard to work around? That’s entirely the point of unknown unknowns - you don’t know what you don’t know.
deleted 8y ago
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