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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.
Pandrol clip
https://en.wikipedi...rol-on-concrete.jpg
Insulated track fixing [Frankx, Oct 04 2019]
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That was some quick looking-up and typing. |
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You could use commuters; they're extremely accurate at detecting and reporting when a train isn't there. |
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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 ... |
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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. |
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*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. |
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[not] - it’s been on my list for a while. |
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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? |
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^ kind of a personal question don't you think? |
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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 . |
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//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 |
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[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! |
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// ‘Balises’ […] only tell the train it’s [sic] current location as it passes // |
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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. |
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There are big problems with GPS, from low obscuration (cuttings) and high obscuration (tunnels and overbridges). |
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The crucial factor is that trains cannot stop quickly. |
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//what the point is of telling the train its current
location. Surely the driver knows// |
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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. |
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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. |
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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. |
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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. |
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So there’s advantage in having a reliable, safe,
train-mounted system that can “read” track status
and occupancy ahead. |
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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. |
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...and less track-based infrastructure, so cheaper
railways, fewer “maintenance hours” etc. |
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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. |
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Trying to super simplify, would track markings, and cheap visual processing, do the job? A tinie bit like a stepper motor. |
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Yes, there is, in the Pilbara. But the trains are, as you point out, relatively few, fairly slow, and exclusively freight. |
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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". |
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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. |
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I believe putting your ear to rail is one way to
detect a train coming, but only once. |
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Indeed; one of many systems where the detection is 100% reliable, but ultimately of doubtful utility. |
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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. |
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I’m an American. What are these things you call
‘trains?’ Are they friendly? |
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Based on the abundant evidence from grade crossing cameras, no. |
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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. |
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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. |
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