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WMATA crash & track circuits

In June, DC Metro had a high speed crash when an inbound Red Line train hit a stopped one. Nine died; about 80 were injuried.

Early on, suspicion has focused on the AC track signals used not just there but worldwide. Such have been around for >100 years.

Track signals work by applying a current-limited voltage between the rails at one end of a block, and having a relay across them at the other end. When a train axle shorts the rails, the relay drops. Outside of very rusty rails, the system is considered VERY reliable; in the fail safe direction. [If it rains too hard, or a rail breaks, or....it shows as occupied. That's OK; but NOT showing a train can be and has been fatal.]

On traction power [third rail or catenary power] systems; the signaling is low frequency AC, with a "WeeZBond" low pass filter used to pass the traction power return to the substation but block the signaling frequencies.

The NTSB has just issued an urgent interim recommendation on such based on their work to date.

The letter discusses the failure they found:

"Testing found that a spurious high-frequency modulated signal was being created by parasitic oscillation from the power output transistors in the track circuit module transmitter. This spurious signal propagated through the power transistor heat sink, through the metal rack structure, and through a shared power source into the associated module receiver, thus establishing an unintended signal path. The spurious signal mimicked a valid track circuit signal. The peak amplitude of the spurious signal appeared at the correct time interval and was large enough to be sensed by the module receiver as a valid track circuit signal, which energized the track relay. This combination of an alternate signal path between track circuit modules and a spurious signal capable of exploiting that path bypassed the rails, and the ability of the track circuit to detect the train was lost."

It's interesting to look at how even a time-proved, widely used, system can fail in an unexpected way....and is a cautionary note for designers of all kinds.

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Reply to
David Lesher
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The original BART system used on-train state-of-the-art (at the time!) redundant digital controls and safety logic. All nice synchronous logic, all clocked by a single crystal oscillator.

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John

Reply to
John Larkin

BART did all kinds of non railroad standard things, such as wider track gauge, higher voltage [1000 VDC] third rail, and their own track signals, controls, etc.

While they had valid engineering reasons for many decisions [They needed wider gauge for stability while crossing the Golden Gate, the 1KV gave more power and less drop, etc.]; they often came back to bite them. Marin County opted out; so BART never ran across the GG Bridge. The motor manufacturers were not prepared to make traction motors that ran at 125% the voltage others did; and oh yes, the control system.

Learning from that; WMATA is built {mostly} with bog-standard railroad equipment; that makes this failure all the more scary.

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Reply to
David Lesher

Why on earth did they never consider a system that has worked reliably for many decades in Europe, Indusi?

No relays, contacts or stuff that can corrode. There aren't too many explanations in English about it but this link contains one on page 3:

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It says "new" while in fact my late grandpa (a steam locomotive engineer in Germany) explained this system to me when I was a kid. If a train engineer screws up the emergency brakes are automatically applied and the train comes to a dead stop. It would have saved many lives in the US.

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Reply to
Joerg

Perhaps because track circuits have been working in the US for well over

100 years? (AC ones for 100+) 

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Reply to
David Lesher

May I quote from your initial post?

Quote "It's interesting to look at how even a time-proved, widely used, system can fail in an unexpected way....and is a cautionary note for designers of all kinds."

On the crash in L.A. there probably wasn't any such system present. There is no excuse for that, and IIRC that case alone has cost 25 lives.

[...] 
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Joerg

The Indusi system as described in the article you cite appears to be a unidirectional "signal -> train" communication system. It looks to me as if it depends on each individual signal source (e.g. road crossing, train detector) working reliably, by itself... if it does, then it ought to be pretty reliable in doing what it does.

It does not, by itself, seem to deal with the question of the reliabiliy of the signal sources themselves - e.g. "train still on the tracks" detection. It doesn't seem to be a detector... just a communicator.

That latter aspect of the problem (the detector) is apparently what failed in the WMATA crash. The "is there a train on this section of the track?" detection process failed, due to the parasitic oscillation and the undesired signal path within the detection system.

For this reason, it looks to me as if adding the Indusi system would not have prevented this crash. It would have ensured reliable communication from the "Is there a train on the track ahead?" sensor to the train... but since the sensor system was malfunctioning and "believed" that the track was clear, there would have been no STOP or EXPECT STOP signal through the Indusi pathway, and the rearmost train would still have been traveling at full speed.

It wasn't that an "engineer screwed up" - it's that the detector screwed up, and gave the engineer wrong (but apparently authoritative) information.

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Reply to
Dave Platt

I only know descriptions in German which won't help here. One component of the German system is the inductive axle counter. If 28 axles have entered a track section and any number smaller than 28 has left it then this section will remain blocked by Indusi. They don't like to rely on anything with electrical contacts over there, and I certainly wouldn't either.

Not on this one. That was in L.A., IIRC he was text-messaging and then crashed. A properly installed Indusi system would have saved 25 lives. Most likely also in DC because the "section occupied" information would have been inductively picked up and registered.

I guess now they'll investigate whether the transistor stage oscillation was a one-time fluke or a design error, and (hopefully) why the track relay circuit was susceptible to a high frequency signal that it shouldn't be listening to. Hopefully the parties involved have good insurance.

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Joerg

..

NTSB will be the first folks to avoid jumping to absolutes before they finish.

First, Sorry.. I forked the thread; I intended one post for a rail group; a longer one in SED.

I suspect you'll find many many installations are getting inspected...

Reply to
David Lesher

Chatsworth was a commuter railroad, rather like the German S-Bahn but it shares trackage with freight.

The Metro is a third-rail subway/surface system with dedicated RoW, AND a dedicated Automatic Train Operation system; more like U-Bahn.

True, it depends on track circuits not axle counters, but then, so does every railroad in this country.

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Reply to
David Lesher

On a sunny day (Thu, 24 Sep 2009 15:22:29 -0700) it happened Joerg wrote in :

The problem was caused by failure to detect the presence of a train. This Indushi seems to be about approaching a spot What happens if there is a power failure with those magnets (cut cable for example). And the train is a diesel-electric or the main power is stil present? It will keep running at full speed past those magnets, Even if there is a train standing still ahead.

Reply to
Jan Panteltje

AFAIK the S-Bahn over there has the same safety fetures as the "real" railroad.

I don't know how the subways in Europe are run, probably different systems.

Accidents like this would be a good time to re-think this. Regardless of how long it has been in operation. Eventually we might also move forward in the direction where other countries have gone, bullet trains and such. At those speeds a crash would have truly catastrophic results. Like the one near Eschede in Germany where a metal rim came off and caused a high-speed derailment.

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Reply to
Joerg

example).

That's taken care of by a magnet that can then not be deactivated anymore. Sorry, guys, this one I can only find in German. Jan, you can probably understand:

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Also, cut cables and vandalism are easily detectable, most of the European railroad runs on its own 16-2/3 Hertz power grid and electric trains will stop when that fails in some area. IIRC they have 100h or so backup power so they could theoretically keep some Diesel traffic but there aren't many Diesel locomotives left.

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Reply to
Joerg

On a sunny day (Fri, 25 Sep 2009 07:33:36 -0700) it happened Joerg wrote in :

example).

Ok, I see.

I think yo uare abit susty on tha trailroad power picture, may I suggest reading:

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Here and in Germany, Belgium, France, large parts of Europe 50 Hz 25 kV overhead is used as standard:
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As far as frequencies goes, seems US is a bit of a minority with 60 Hz mains:

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Reply to
Jan Panteltje

example).

reading:

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overhead is used as standard:

Nope. Read up on it (sorry folks, it's in German as well):

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Quote: "Deshalb wurde die Frequenz auf ein Drittel, das heißt auf 16 2/3 Hz, reduziert. Diese Frequenz ist in Deutschland bis heute gültig, wenn sie auch inzwischen auf 16,7 Hz angehoben wurde."

... which in essence says that they chose a frequency of 1/3rd of line frequency and that has been kept in place until today, except that they went from 16.666Hz to 16.7Hz.

With 50Hz they had too much motor damage when accelerating large trains from zero (when stopped).

AFAIK there are no electrified long distance tracks in the US. That's all Diesel. Commuter trains are sometimes electric.

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Joerg

On a sunny day (Fri, 25 Sep 2009 13:07:44 -0700) it happened Joerg wrote in :

'gültig' is more like 'in use', available, but not let's say 'for new designs'. From

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gültig = valid, valid, validly, available, validated

Well 50 Hz 25 kV is a European standard, and in Germany used here: Germany Rübelandbahn Harz If you look here, and scroll down to 50Hz 25 kV, you see where those systems are in use.

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Well, that is old stuff, it is all solid state converters and control these days, In my country you could (can) hear the thyristor switches make the inductors singing when the trains started from the station.

We now get high speed (line to France) and that is all 25 kV 50 Hz too. In the modern age there really is no need for a separate frequency. This is fun, and much of the future:

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They use 3 phase synchronous motors:

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So, 50 Hz, whatever, is converted to a different frequency to drive the motors. I am no train expert, but this makes a lot of sense to me. Having a separate power station with a different frequency just for trains (and at 16.x Hz transformers would be idiotic big) must suck in so many ways that you wanted to get rid of those immediately.

Reply to
Jan Panteltje

It is still the standard. Changing it would require _huge_ investments. Why should they?

That's a li'l choo-choo train :-)

I am talking real railroad. Long-haul freight, ICE bullet trains and such. Not some boutique track.

are in use.

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Yeah:

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Not exactly the mainstream German railroad, is it?

days,

singing

Keep in mind that locomotives are used for at least as long as aircraft.

30-40 years is nothing, it is very typical. When I was in Germany last year I saw some of the very same locomotives that I saw when I was a child, and even then they weren't new. Like this model, in use since the 50's:

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When I have to design for this market I am not at all surprised when the client tells me that it has to be designed for 30+ year service life. Meaning no electrolyics etc.

They are happy with it since many decades. Old rule: If it works, don't try to fix it.

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Reply to
Joerg

Because modern VFD-AC motors provide better traction, far lower maintenance, better efficiency, and weigh less then 100 year old DC motor designs.

Are you sure someone hasn't just rounded things off?

We're still stuck with some leftover 25Hz on the Northeast Corridor but they'd LOVE to replace that, if you'll send them a check...

Low freq AC is a vestige of the rotary rectifiers and similar antiques era.

&

Do you call the Northeast Corridor a boutique track? I wonder where its daily traffic would put it vs your "real" railroad examples? [I really don't know...]

days,

singing

True...

Reply to
David Lesher

In old days various motors with commutators and adjustable field windings were used for variable speed operation. These worked of course well on DC and in trains often with the possibility to connect in parallel or series for even large speed range. Such motors can also be used on AC with voltage (phase) control e.g. in small hand tools at 50/60 Hz.

However, large AC/DC motors suitable for train operation did not work very well on 50/60 Hz and this appears to be the reason for using such low line frequency as 16.67 Hz in trains in some countries. Compared to DC feed, in the AC feed, the operating voltage could (instead of a variable resistor) be varied by selecting a different tap from the on board transformer.

While it would be impractical to change a railway line voltage or frequency, building new trains with a variable frequency drive and induction motors would be more practical these days, possibly even replacing the heavy 16.67 Hz transformer with a rectifier+inverter and a high frequency transformer on the 15 kV side.

Paul

Reply to
Paul Keinanen

But when the old one hasn't been amortized yet or the bean counters say the new investment has no ROI then ye olde motor will remain inside the locomotives and be repaired over and over again. Very normal in the railroad business (I knew some guys who worked there). As for efficiency the difference isn't that smacking and power is fairly cheap.

AFAIR it was a big thing over there, lots of discussions. Beats me why.

Listen to a freight train accelerating over there. Pock ... clonk ...

*POCK* ... tock ... tons of huge contactors inside the locomotive ratcheting the settings. Yeah, those are sometimes really old locomotives but they still work so they won't scrap them.

Ok, there are some locally. But not across the whole country.

days,

singing

There is no need, but there is also no money to buy all new stuff with marginal or no ROI versus the old stuff.

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Joerg

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