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Train detection system

Time-domain reflectometry to know where trains are
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Signalling systems stop trains from colliding. Which is good. Conventionally, they use (amongst others) ‘track circuits’ and ‘axle counters’ to determine where trains are - which is important when you’re keeping them apart. Unfortunately, GPS isn’t precise enough, because you need to know location at metre or sub-metre levels with high reliability and integrity. And in tunnels.

Newer systems (communication -based train control and ETCS) use ‘Balises’ (passive transponders in the track- bed) as absolute location references - but these only tell the train it’s current location as it passes, it’s not continuous.

Time-domain reflectometry is a really nice technology - effectively “radar down a cable”. A pulse/signal is injected into one end of a cable under test, and as it propagates down the cable, it gets partially reflected by the features it encounters as it travels. And the reflected waveform has a characteristic pattern (which tells you what kind of fault you’ve got) and a flight time (which tells you where the fault is). This is mostly used for telecommunications cables (both for optical and conductive carriers)

This idea is to use a train-mounted time-domain reflectometry system. The train injects a signal into the rail (inductively or conductively) and listens for the returning signal. Should be able to detect a train in front, set of points, or rail defects ahead. Multiple send/receive heads (perhaps at either end of the train) allow “in- front” and “behind” to be distinguished.

Injected signal could be digitally encoded with information about the current train.

Highest resolution/greatest sensitivity is nearest the train, where it’s most important.

Frankx, Oct 03 2019

Pandrol clip https://en.wikipedi...rol-on-concrete.jpg
Insulated track fixing [Frankx, Oct 04 2019]

[link]






       That was some quick looking-up and typing.
notexactly, Oct 03 2019
  

       You could use commuters; they're extremely accurate at detecting and reporting when a train isn't there.   

       OK, so, yes, absence of evidence and all that ... but a platform full of people all screaming "WHERE'S OUR BLOODY TRAIN ?" is a useful clue ...   

       Unfortunately the same system could not be applied to buses. In rural areas of the UK, the only travellers who have actually seen a bus are typically elderly and with poor eyesight. To anyone under 60, a bus is something immobile* that you look at in a museum.   

       *It has been pointed out that some museums, such as the Black Country and the Beamish ones, actually do have antique buses that transport visitors around the site; so they are not invariably static exhibits.
8th of 7, Oct 03 2019
  

       [not] - it’s been on my list for a while.
Frankx, Oct 03 2019
  

       Oh—it seemed that you posted it after seeing my mention of TDR on the other idea just a few minutes earlier. How long is your list, by the way?
notexactly, Oct 03 2019
  

       ^ kind of a personal question don't you think?   

       It seems to me that since the track is grounded with railroad spikes on every tie, the instruments for TDR in this application will need to be much more advanced than standard TDR in an insulated wire. It might work slightly since the track has a lower resistance than the earth ground, but .
scad mientist, Oct 04 2019
  

       //lower resistance// Modern rails usually have somewhat insulated fixings [link], but even older ones are dependant on a relatively higher resistance to ground for track circuits to work. Track-bed resistance is a well-studied phenomenon, shouldn't be a problem in this application, it'll dictate the relationship between the range and the energy injected/recieved
Frankx, Oct 04 2019
  

       [not] - I saw that idea and recognised it was describing TDR, and remembered that I had been sitting on this idea for ages. My "list" isn't actually written down, so I have no idea how long it is!
Frankx, Oct 04 2019
  

       I remember when this job wasn't automated. An army of volunteers called trainspotters would use CB radios to call in location updates.
bigsleep, Oct 05 2019
  

       // ‘Balises’ […] only tell the train it’s [sic] current location as it passes //   

       Are you sure? I don't see what the point is of telling the train its current location. Surely the driver knows, and if the train itself really needs to know, a GPS receiver would be cheaper and better. I would find it more plausible that the train's location is reported to the control center, but I think what I've read in the past agrees with your claim, bizarrely.
notexactly, Oct 05 2019
  

       There are big problems with GPS, from low obscuration (cuttings) and high obscuration (tunnels and overbridges).   

       The crucial factor is that trains cannot stop quickly.
8th of 7, Oct 05 2019
  

       //what the point is of telling the train its current location. Surely the driver knows//   

       Modern railway systems strive to reduce reliance on the driver. The signalling & train control system needs to know the location of every train with high precision and high integrity. If it “loses track” of a train, bad things happen.   

       Conventionally, that’s done by equipment mounted on the track - but that has disadvantages: every km of track needs expensive equipment, and it all needs to be maintained regularly, which means shutting the railway for maintenance.   

       And when it fails, lots of passengers are affected, often for long periods of time - “train delays caused by signalling failure at...” are all-too- common words.   

       Trains, as [8th] said, take a long time to stop. A normal passenger train might be 400-500 tonnes, travelling at 125mph (say 55m/s). Relying on drivers alone (called on-sight driving) would limit trains to very low speeds, and very large distances between trains, for safety.   

       So there’s advantage in having a reliable, safe, train-mounted system that can “read” track status and occupancy ahead.   

       Distributing critical track occupancy calculation to trains would mean that a fault would affect a single train and those around it, rather than a wide area.   

       ...and less track-based infrastructure, so cheaper railways, fewer “maintenance hours” etc.   

       GPS is used, in a small number of cases- I think there’s a big mining railway in Western Australia that uses GPS for train position. But that’s a very different environment from a crowded urban metro system.
Frankx, Oct 05 2019
  

       Trying to super simplify, would track markings, and cheap visual processing, do the job? A tinie bit like a stepper motor.
wjt, Oct 05 2019
  

       Yes, there is, in the Pilbara. But the trains are, as you point out, relatively few, fairly slow, and exclusively freight.   

       Operating a single track line that way is entirely practical and "no less safe" than traditional methods. But for an urban commuter system ... "we see dead people".   

       Line of sight driving is incompatible with very high speeds, but conversely the technology is only an aid to the driver where the block ahead is occupied; automated systems can't yet replace human eyesight or threat detection.
8th of 7, Oct 05 2019
  

       I believe putting your ear to rail is one way to detect a train coming, but only once.
not_morrison_rm, Oct 06 2019
  

       Indeed; one of many systems where the detection is 100% reliable, but ultimately of doubtful utility.   

       The "undercarriage retracted" warning sound on aircraft - an unmistakable noise of tearing metal - is also 100% reliable, yet has never gained wide acceptance in the aviation community.
8th of 7, Oct 06 2019
  

       I’m an American. What are these things you call ‘trains?’ Are they friendly?
RayfordSteele, Oct 07 2019
  

       Based on the abundant evidence from grade crossing cameras, no.   

       Trains in the U.S.A. are primarily an efficient means of eliminating careless or unwary drivers of road vehicles (usually trucks) from the general population.
8th of 7, Oct 07 2019
  

       // Pandrol clip link //   

       The other picture in that article is of a sculpture of a Pandrol clip in Calgary, where I happen to live. I've visited it, specifically and deliberately, several years ago, despite not being especially interested in trains, because it's an Ingress portal. I also read that Wikipedia article around then, but I don't remember if that was for a related reason or not.
notexactly, Oct 07 2019
  
      
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