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Hong Kong MTR trains collided when testing SelTrac CBTC

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Hong Kong MTR trains collided when testing SelTrac CBTC 20/03/2019 at 18:27 #116470
alvinhochun
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225 posts
News article from SCMP, photos included.

I thought this might be of interest to some of you here (if you haven't already heard of it through other channels), as this is a catastrophic failure of a modern signalling system. Luckily it's only a test and no passengers are harmed (though a train operator did get injured). The aftermath took two days and nights to clean up

Short version of what happened: A train was leaving the terminus station platform to use a double crossover to get to the opposite track. Another train approached the station, aimed for the opposite platform and entered the double crossover while the first train was crossing. The end result is a side collision with derailment.

The most interesting part of this event is a picture (attached) of the signalling screen that circulated on the Internet after the incident. It appears to show two conflicting routes being set, as indicated by the two paths with arrows along them (I believe this is how the screen works).

It is said that Thales (the software supplier) was able to reproduce the incident with their simulation and thought it would be a software bug. I'm in disbelief that a software bug could cause the interlocking to be ignored. Completely unacceptable for a safety-critical system. Good thing they managed to catch it during testing! But seriously, they should've caught it during earlier simulation runs, given that they could reproduce it afterwards.


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Hong Kong MTR trains collided when testing SelTrac CBTC 20/03/2019 at 19:30 #116475
madaboutrains
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Visually that is the same signalling system used on the Jubilee, Northern Lines and as of Monday Morning Hammersmith to Latimer Road.
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Hong Kong MTR trains collided when testing SelTrac CBTC 20/03/2019 at 20:23 #116477
Edgemaster
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madaboutrains in post 116475 said:
Visually that is the same signalling system used on the Jubilee, Northern Lines and as of Monday Morning Hammersmith to Latimer Road.

Sunday afternoon, I had a ride on it shortly after opening for public service

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Hong Kong MTR trains collided when testing SelTrac CBTC 20/03/2019 at 20:41 #116478
Jan
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madaboutrains in post 116475 said:
Visually that is the same signalling system used on the Jubilee, Northern Lines and as of Monday Morning Hammersmith to Latimer Road.

Also the trunk line of San Francisco's Muni Metro, which uses Seltrac, too.

Two million people attempt to use Birmingham's magnificent rail network every year, with just over a million of them managing to get further than Smethwick.
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Hong Kong MTR trains collided when testing SelTrac CBTC 20/03/2019 at 21:29 #116479
GeoffM
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To be fair, there are several versions of SelTrac. One media report stated that the Singapore metro (or part of) would need the same fixes, but did not mention other installations, whether through a lack of knowledge or because it's a different version, I don't know.

On a side point, that scissors failure looks like an interlocking logic issue, not a software problem. But then some people refer to data as software so who knows. In the UK it would not be possible for all four point ends to be reversed with a route over any part, which nicely handles the diamond in the middle issue.

SimSig Boss
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Hong Kong MTR trains collided when testing SelTrac CBTC 21/03/2019 at 08:28 #116483
alvinhochun
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GeoffM in post 116479 said:
To be fair, there are several versions of SelTrac. One media report stated that the Singapore metro (or part of) would need the same fixes, but did not mention other installations, whether through a lack of knowledge or because it's a different version, I don't know.

On a side point, that scissors failure looks like an interlocking logic issue, not a software problem. But then some people refer to data as software so who knows. In the UK it would not be possible for all four point ends to be reversed with a route over any part, which nicely handles the diamond in the middle issue.

Right. There are even several types of technologies used. The earlier SelTrac S40 uses interleaving transmission loops for both localization and data transmissions. This is the system used by the DLR, Jubilee line and Northern line if I haven't mistaken. The same system has also been used for a major rail line and a branch line in Hong Kong for a decade and a half. Some people apparently refer to this system as TBTC instead of CBTC.

The failed signalling in question that MTR installed is the newer wireless CBTC one, not the loop-based one. From what I've heard it seems that the 4LM project is using a similar system.

The spokespeople was saying that the incident happened when they switched to the "third backup system" (which they said is unique to MTR) as part of the test, so it could be that the switchover somehow caused issues. It may be irrelevant to other SelTrac CBTC systems around the world without the "third backup system". But then, PR speak isn't the best source for technical information. Same for the media. The local news are very bad at understanding the technical stuff.

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Hong Kong MTR trains collided when testing SelTrac CBTC 21/03/2019 at 08:44 #116484
kbarber
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So what is the third backup system and where, exactly, does it fit in the overall logic of the system?

As Geoff says, what we see in that image is undoubdtedly an interlocking failure; even if the crossovers are not locked against each other (and with the UK history - and, I believe - a tradition of UK managers in MTR's hierarchy that continues to this day I would have expected MTR to follow UK practice), the signals most certainly should have been.

Incidentally, Modern Railways carried a report that a software issue emerged during testing between Hammersmith and Praed Street, not safety critical (if I read the report correctly) but which they decided should be fixed before the system was commissioned, hence the long delay in commissioning. (It was originally supposed to be last summer if I understand correctly.)

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Hong Kong MTR trains collided when testing SelTrac CBTC 21/03/2019 at 15:06 #116488
metcontrol
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The core system of both the Hong Kong and current LUL installations is the same, however what we have is different in certain aspects. The newest installation (on 4LM) is even different to that on the Jubilee and Northern - though from an operator perspective they look fairly identical. Hong Kong opted for the version where trains can literally go anywhere - backwards and forwards over the same piece of track / over points in any direction. We opted for a more restricted version, where certain locations have more flexibility but on the whole trains on say, the eastbound, will always operate in that direction. In the event of failure, we still need to employ safety procedures to run trains in the "wrong" direction.

The main difference, as far as LUL is concerned, between TBTC and CBTC is the way in which the system talks to trains. The Northern and Jubilee use TBTC which speaks to trains via wired loops fixed to the track. CBTC used on 4LM uses radio beacons rather than wire loops to speak to trains.

The main part of the system runs the service and keeps trains apart, monitors headways, and generally does what you would expect from such a system. However, on LUL, underneath the automation we still have a system of fixed "track circuit blocks." I use the term loosely as they are not wired in the same way as the old signalling (most are axle-counter blocks) but they perform much of the same job. They are the underlying safety system - the back-up insurance policy if you like. If they "fail" we can still use the CBTC system safely to work around the failure until they are fixed, albeit with restrictions.

The first part of the 4LM commissioning went ahead on the weekend - it was in fact almost exactly a year late. There were some issues found during the testing process - some potentially serious if the right circumstances all happened to come together. All centred around operations during a failure situation and none were connected to the day-to-day running. These have been cured as far as possible and where the fix hasn't been all that was desired, certain restrictions have been put into place. I cannot obviously go into any further detail.

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