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Comment by Terr_ | Hacker News Reader

I hope they do a followup about what kind of problem they found in PV46.

I suspect that someone flipped the switch away from "More Magic" :)

http://catb.org/esr/jargon/html/magic-story.html

I've been caught out by "ground" not really being ground before; two different sides of a whole floor computer / comms centre had different potentials. So most of the power was hooked up to ground on one side of the building and a small number of non computer / comms gear was hooked up to ground on the other side (think lights etc that were too difficult to rewire), but the other gear used wall sockets that were a different colour to make sure people didn't plug computers or network gear into them. This was all discovered after a temporary 20V spike difference in the different grounds trashed an expensive router. The electricians suspected that because we used a largish number of microwave links on the building the copper bars that run down the building to ground were acting like antennas and picking up the energy. So we defaulted all the sensitive gear to the "ground" that was least problematic. It was cheaper than re-doing all the ground circuits in the building with RF shielding. The joys of old buildings.

A popular train line out of London has an electrical problem like this with one of the train models run upon it. Whenever these trains, travelling about about 100mph, cross from one power section to the next with the front the train entering one, and the rear leaving the other, there is some weird electrical event that causes a safety mechanism to violently retract the rear pantograph[1] (maybe more than one). This causes an almighty WHACK on the roof which scares the bejesus out of any visitors not used to impact noise. I've seen at least one coffee ejected from a cup onto someones trousers because of this. The train progresses unharmed.

[1] https://en.wikipedia.org/wiki/Pantograph_(transport)

Are you sure this isn’t just the pantograph passing through the neutral section? They have these on long runs where the power is supplied from different sources (resulting in sync issues). There are circuit breakers at these points which on some trains makes a very loud ‘clunk’, which stops the train trying to draw current through . The other type of ‘clunk’ you get on some trains (although not the ones you are describing) is the pant going up/down on combined AC/DC lines.

It could be circuit breakers, yes. They would need to be big devices, though. It is very loud. Best I can come up with is someone dropping a shopping trolley on the roof from a storey or two high. I got the pantograph information from a rather "train-nerd-I-love-my-job!" sounding driver over the tannoy one evening perhaps two years ago. I may have misheard!

They are the circuit breakers. Our trains in Brisbane, AU run 25kV (Thameslink seems to be the same 25kV overhead, with a 750V third rail, but this will be the overhead section) and the circuit breakers trip during the entry/exit of a neutral section, causing an almighty bang as they kick out and back in. If you're sitting in the wrong carriage directly underneath them it's very loud.

For reference, 1kV arcs across about 1cm of air, so 25kV needs a full foot of space in air (obviously less with a dielectric, but you get the orders of magnitude).

Source: I design trains for a living, though on the mechanical side

Some trains have mechanical contactors (https://en.wikipedia.org/wiki/Contactor), they can be very loud when switching.

Can you say which train line? I use C2C and I've heard occasional loud bangs that scare people, but never know what causes it.

Thameslink, and only in these: https://en.wikipedia.org/wiki/British_Rail_Class_377

This also happens on the Stansted Express. [1]

[1] https://twitter.com/stansted_exp/status/527011030728441856

I've noticed this phenomenon while riding Greater Anglia Class 379s on the WAML. It's very annoying!

This would make for a really great blog post! Could you write it up in greater detail?

According to the linked press release[1]:

> ... faulty train signalling hardware on PV46 was emitting erroneous signals in addition to the ones it was supposed to emit. These erroneous signals occasionally prevented trains travelling in the vicinity of PV46, including in the opposite direction, from properly communicating with the trackside signalling system. This loss of communications led to the activation of the trains’ emergency brakes.

[1]: https://www.lta.gov.sg/apps/news/page.aspx?c=2&id=89defe38-4...

Thanks! I was like, I got to the end of the article and they didn't tell us what the issue with the train was. :-P

Sounds like it may have been sending an (off-by-one, inverted, etc) signal indicating it's on a different track, or moving a different direction or speed, than it was in reality.

While the press release says these were "in addition to the ones it was supposed to emit", it's not hard to imagine that a similar error could happen where the wrong signals were sent in-lieu-of the intended signals, and thus leave trains rolling that should have detected an emergency-stop situation.

They might have problems reproducing such an intermittent problem without running the train. Often times such a problem clears up when someone physically messes with the hardware. Under such circumstances the problem would probably be solved by taking the circuit board that does the communications out and then carefully placing it in the garbage, possibly physically destroying it first.

Not all problems can be troubleshot and there are some things that remain mysteries forever.

It would also be interesting to see how the nature of the cause can explain the one statistical surprise that remains: PV46 is no outlier at all in figure 3, "frequency by train ID". According to the preliminary explanation, PV46 should either be so massively overrepresented there that the bughunt would be over before it really started (it is always close to itself), or it should only be affected by the estimated 5% of outages not related to PV46 which it clearly isn't.

Except that figure shows the frequency with which PV46 experienced an interruption, not the frequency with which it was near an interruption. As stated in the article, "We could not inspect the detailed train logs that day because SMRT needed more time to extract the data", implying that there was no position information for trains that were not actively experiencing a disruption.

Arguably, based on the preliminary explanation, it seems like PV46 might be over-represented (it had 4 interruptions) -- after all, it never travels in the opposite direction to itself.

It's in the press release linked from the article.

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