Croydon decoupling report

 
  bramt Deputy Commissioner

ATSB / CITS investigation report into decoupling at Croydon last year: https://www.atsb.gov.au/publications/investigation_reports/2018/rair/ro-2018-019/
I was interested to read about the functioning of the emergency brake safety loop circuit:
When the safety loop is opened due to an abnormal operating condition, the emergency brake solenoid valve is de-energised causing a maximum brake application.
ATSB
I wanted to know why the lead 3 car xtrap's emergency brakes didn't apply after the break. My understanding is if a loco-hauled train breaks apart at any point, brake pipe separates, emergency brakes come on on all wagons, both behind and ahead of the break.

In this instance, the uncoupling function was activated through an electrical circuit that was created by a wiring error in car 959M and the low IR of the uncouple solenoid of car 882M. This error resulted in the cab active relay, push button and the 3 km/h relay interlock devices being bypassed and allowing the unintended activation of the uncoupling circuit. The safety loop circuit on the lead car unit maintained the closed safety loop, hence emergency braking was not applied and it continued its journey until the driver realised that the trailing car unit had uncoupled, and brought the lead 3-car unit to a stop. As there were no active cabs in the trailing 3-car unit, the safety loop was opened, de-energising the emergency brake solenoid valves, resulting in the application of the emergency brakes on the trailing car unit bringing it to a stop.
ATSB
There's no analysis of whether the loops on the 2 3 car sets should be independent, whether losing air pressure should apply brakes on both sets, or whether the emergency brakes should have come on the lead car at all.

Should it?
If a Comeng separates at the Scharfenbuergs while at speed, do the brakes come on on both sets?

Or is it a case that due to the wiring fault, the lead xtrap thought it was supposed to be decoupling - and the way you do that is by moving away from the other set - meaning emergency brakes should not be applied?

Sponsored advertisement

  Valvegear Dr Beeching

Location: Norda Fittazroy
All of this hoo-hah is what happens when a simple and reliable system is replaced by something complicated.
The old system of automatic couplings and Westinghouse automatic air brake would have stopped both halves of the train at once. It was genuinely "fail safe" where obviously the current system isn't - the three leading cars continued on their merry way until the driver "realised" that half his train had gone missing.
  route14 Chief Commissioner

The report even had the incident time wrong at the beginning of the article, looking as if the train had traveled back in time.
  lkernan Deputy Commissioner

Location: Melbourne
Considering the same technician had incorrectly wired 11 sets, they're lucky trains weren't spontaneously splitting all over the network.
  hbedriver Chief Train Controller

ATSB do not always a great report, although this appears to be one of their better ones.

Note that an X'trapolis does not run with a positive brake pressure (only atmosphere pressure), thus zero brake pipe pressure per gauges, or in certain operations coupled to traditional trains (eg loco hauled to Ballarat workshops) they run 550kpa to have a compatible brake.

The X'trapolis works through the (electrical) safety loop, and is usually fail-safe (any open circuit applies emergency brakes), although as remarked in the findings, Metro are talking to the builders about that one. Things like low voltage in the batteries will stop the train (before the volts get too low to apply the brakes), also activation of the trip, Vicers alarm, or various stop buttons. The fact that the rear portion (which lacked any person on the train to stop it otherwise) stopped automatically is reassuring. The front portion had the driver holding the safety loop closed (through his key and controls), hence it could keep running.

The train received a (false) signal to uncouple, and responded accordingly thinking it was a proper command. In that sense, everything worked as designed.

Note also that this event required two unusual failings, poor insulation in one carriage, and also the wiring fault in another. Had the two units been coupled the other way round, neither fault would have mattered.

One does wonder whether this might affect some of the automated trains running (eg the Sydney metro, trains being built by the same builder as the X'trapolis), or the semi-automated ones such as Melbourne HCMT's. Now, there's a can of worms to open...
  bramt Deputy Commissioner



The train received a (false) signal to uncouple, and responded accordingly thinking it was a proper command. In that sense, everything worked as designed.
hbedriver
Thanks for the informative post. This part makes perfect sense - it would be difficult to legitimately divide the trains if emergency brakes came on both sets as soon as they part. But I wonder if they should still come on if any decoupling occurs at speed. Eg, a serious track fault causes the rear half of a set to derail at speed, separating the train. In this situation, shouldn't the brakes come on both sets? I'm thinking of the Eschede disaster, the emergency brakes still applied on the lead cars (although the ICE didn't have intermediate auto couplers)
  bramt Deputy Commissioner

The report even had the incident time wrong at the beginning of the article, looking as if the train had traveled back in time.
route14
Nope, it left Camberwell at 1552 in order to operate the 1607 Flinders St - Lilydale service.
  route14 Chief Commissioner

In the first line of the second paragraph in "Summary", it indicated 15:57 as the incident time.
  hbedriver Chief Train Controller

Thanks for the compliment, BRAMT.

There were safety features to guard against unintended uncoupling, but the insulation failings allowed them to be by-passed. There was then a false uncouple command, activated by a wiring fault in another carriage.

The event is deemed to have occurred while separate faults on different carriages occurred simultaneously, usually something you cannot really predict. Although whether a train with a positive brake pipe pressure (eg, Vlocity or Sprinter) would have had the same result is another matter; the answer to that is I don’t know, I only drive them!
  route14 Chief Commissioner

Coincidentally, trains nowadays seem to use low note whistle to go through level crossings.  It used to be a long high note whistle.

Sponsored advertisement

Display from: