Is Longrider about? How does this work then?

To call on this blog’s rail signalling expert. How in buggery is this happening?

Newly-refurbished trains have been pulled from service over fears they are accidentally turning signals red or yellow.

South Western Railway (SWR) reintroduced the 30-year-old Class 442 trains in June after giving them a £45m upgrade.

But they have now been taken out of service as a precautionary measure amid concerns they are adversely affecting line-side signals.

We can handle technical details if they’re known…..

17 thoughts on “Is Longrider about? How does this work then?”

  1. They are suspected of accidentally turning signals yellow or red as they pass through

    Isn’t that how it’s supposed to work? Train goes through green, enters the block, then the signal turns red to indicate that the block is occupied.

    Presumably the problem is that the signals aren’t turning green again when the train leaves the block. (Yes it’s more complex what with moving-block signalling these days, but the principle is the same.)

  2. The point of asking Longrider is that he teaches this stuff – signalling – to people. We hope to find out!

  3. My previous life was was Driver Training on Alternating Current Electrical Multiple Units.
    This isn’t new, there were similar problems when Liverpool Street was re-signalled in the late 1980’s.
    Without trying to be too techie, grey Location Boxes (‘LOC’ is painted on the outside along with a ref. number) are situated along the track which contain relays / modules etc. concerned with the signalling system.
    Now although there should be a nominal 25Kv on the overhead line it varies quite a lot (it is common on Class 360 EMU’s to see 23000 volts on the train management system), so modern trains take the nominal 25KV Alternating Current from the overhead line, convert it (rectify) to Direct Current, then reconvert it back to AC. This ensures the traction motors and other systems on the train gets exactly the voltage and frequency they require. This can create loads of electromagnetic interference though which can screw up the relays etc in the location boxes when the trains pass.
    The trains should be tested to see how much interference they generate before being put into service to prevent this happening.

  4. Without more information, I believe that Addollf nails it. My first thought was interference rather than track circuitry. Not least given that these are being replaced by axle counters anyway.

  5. “……The trains should be tested to see how much interference they generate before being put into service to prevent this happening……..”

    Such testing is a standard requirement in most electrical equipment that I know of – or at least the documentation which comes with each item claims that it is, and that the item has passed such tests.

    So I would be surprised to hear that there is no such specification requirement, or testing for it. I wonder whether the issue is that the levels of Radio Frequency Interference allowed by Network Rail are incompatible with those specified by the train operators? Such an issue might arise if you split a railway up into a lot of separate management units….

  6. Or, of course, the trackside Location Boxes are insufficiently resilient. Usually there are standards for sensitivity to radiated and conducted RF as well.
    (I do electronics but not for trains, although I know it can be a very hostile environment.)

  7. Bloke in North Dorset

    What others have said sounds plausible. Changing AC to DC and then back to AC causes lots of interference and in my day Switched Mode Power Supplies were a nightmare for electro-magnetic compatibility (EMC) and I presume they still are.

    EMC testing is both time consuming and expensive as the testers have to search for spurious RF signals and then suppress them. I presume it is quite automated now but that doesn’t make the test environment cheaper. The act of suppressing spurious RF signals can cause other spurious RF signals at different frequencies, so the whole testing regime has to start again every time they find one.

    By now we should expect Network Rail to have an EMC profile for the boxes and tell the train designers and testers what to look for, but that’s not foolproof.

  8. I’m not familiar with the rail track side industry specifically, just more general industrial stuff….
    But it would be very unusual for RFI from the motor conversion gear to be strong enough to upset electromechanical signalling! Portable jamming indeed!
    Not at all unusual for strong RFI to upset first generation digital kit though, so if the signalling is that, all is explained.

    Electromechanical kit is far more likely to be upset by ground currents…and 500A of return current in the rails from a passing train is a suspect. If the grounding isn’t all it should be, parts of the 0V will be many volts away from other parts of 0V, and that can cause any amount of relay fun.

    No idea what a train return current might be…anyone?
    But in industry, 500A of earth current can give big problems if the grounding has ‘grown’ over the years.

  9. “But it would be very unusual for RFI from the motor conversion gear to be strong enough to upset electromechanical signalling!”

    My thoughts as well

    “Not at all unusual for strong RFI to upset first generation digital kit though”

    In my telecoms career I well remember when carbon granule microphones and dial type phones started being replaced with electronic equivalents. As my “patch” was close to a BBC medium wave transmitter site we soon got complaints from customers who found their telephones were picking up Radio 1&2, making conversation with their callers difficult. And that was simple analogue technology, so it wouldn’t need more than a few diodes incorporated into the circuitry of trackside signalling boxes to cause problems.

  10. Converting AC to DC and back again is not really to do with supply voltage, but a core function of inverter drives. Both the frequency and voltage supplied to the motors will be varied by the control system to suit the speed and power required from them. At a rather more mundane level, we’ve had one on the lathe at home as a better option to a mechanical variable speed drive.
    There’s an inherent regulating function so variations in supply voltage won’t make any difference – but that’s a side effect of the core function.
    On at least one class of train, it would appear that there’s more to it. I’ve heard some of them setting off and there’s a distinctive whine (I assume due to magnetostriction) – and with at least one class of train, the whine changed several times as if changing gear. So I assume they do several steps of motor reconfiguring – it would make sense when a very wide range of speed is needed, more poles for better performance at low speed (otherwise a very low frequency is needed and/or a high slip angle creating large iron losses), fewer poles for higher speeds to avoid very high frequencies.

  11. “the whine changed several times as if changing gear”

    Ah! That explains the noise. I’ve been on trains that sounded like a milk float setting off.

  12. @Tim Worstall September 3, 2019 at 7:38 am

    The point of asking Longrider is that he teaches this stuff – signalling – to people. We hope to find out!

    “We hope to find out” that Longrider teaches signalling to Learner CBT motorcyclists

    Trains are not motorcycles

    @Longrider have I missed a previous career?

  13. This hasn’t come up yet so worth mentioning: the routes that the 442s on South Western Railway run on are all 750V DC third rail routes. Until the most recent refurbishment, the 442s (built in the late 1980s) had DC traction motors that themselves had been salvaged from trains built in the early 70s, so the electrical system was a fairly simple and direct “DC third rail -> pickup shoe -> some wires -> DC motor” setup.

    The rebuilt that the 442s have undergone recently is to replace the 1970s DC traction equipment with modern AC equipment, so the system is now complex in a similar way to how people above have mentioned, but slightly differently: “DC third rail -> pickup shoe -> inverter -> AC motor”.

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