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Anyone know what this is?

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bakerstreet

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Seen here at Mornington Crescent northbound platform but I think I’ve seen them elsewhere too
It’s high up on the platform wall, pointing towards the trains (although where it’s pointing could be irrelevant).
What interests me is it makes a seemingly constant fast and repeated clicking noise, rather like the sound of a quiet gas hob sparker.

1652909105860.jpeg
 
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Dstock7080

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Could it be the device that triggers the platform PA announcements when the train breaks the beam?
 

John Webb

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It's not like any smoke detector I've ever seen. The labels on it tell us nothing useful, alas. I wonder if it is some form of air quality sampling/measuring device as the cylindrical bit on the front doesn't seem to be any form of lens.
 

rogercov

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I think it's a train mass detector (TMD).
I'm not sure what it controls, but it may be to activate the track-to-train CCTV when the train is in the station.
Perhaps someone else can confirm this.
 

Lewlew

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I think it's a train mass detector (TMD).
I'm not sure what it controls, but it may be to activate the track-to-train CCTV when the train is in the station.
Perhaps someone else can confirm this.
CCTV (OPO monitors) usually come on just before the train enters the platform. At least they do on the Central line.
 

100andthirty

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Mass sensors were fitted to turn off the cross track advert projectors. I don't know a) whether this is one of those and b) whether there is a cross track projector at Mornington Crescent!
 

43066

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At a pinch - some kind of theodolite device measuring ground movement?

A lot of them were placed in LU tunnels during the Crossrail tunnelling and certainly looked similar to this. Albeit I don’t think Mornington Crescent was an affected area.
 

Recessio

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At a pinch - some kind of theodolite device measuring ground movement?

A lot of them were placed in LU tunnels during the Crossrail tunnelling and certainly looked similar to this. Albeit I don’t think Mornington Crescent was an affected area.
It's vaguely near some major HS2 sites, but I'd be surprised if they were worried about it from that far away. Probably a good 200m or so away.

(I lived in a council block literally next to the site and they didn't give a toss about us...)
 

swt_passenger

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At a pinch - some kind of theodolite device measuring ground movement?

A lot of them were placed in LU tunnels during the Crossrail tunnelling and certainly looked similar to this. Albeit I don’t think Mornington Crescent was an affected area.
The targets around a station for the theodolites to measure when checking for movement are passive, this thing looks powered.
 

randyrippley

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looks to me like an air sampling device - possibly monitoring CO2 / CO levels.
The soot deposits are a giveaway that it's sucking in air for some purpose
I'd guess that the two probes on the right, poking through the wall, are thermocouples monitoring air temperature
 

vinnym70

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There were (possibly still are) devices on platforms 9/10 at Stratford that emitted a regular click sound. I only ever noticed them because of a fatality which services were just recovering from with significant delays.
It was hot, crowded and the continual clicking noise was something that ended up annoying me more than it should have in the circumstances so I decided to investigate.
They definitely seemed to be related to detecting the presence of a train in the platform and turned on the CCTV cameras that covered the platform.
They were probably 7.5/8ft off the platform level so high enough not to be affected by people walking past them but lower than the height of a train in the platform.
They didn't look like the photo provided but the clicking noise mentioned triggered me to mention this.
 

rogercov

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Just to confirm my earlier post and to kill off any suggestions of it being a ground movement measuring device, it just sends clicks and detects the reflection from a train. It may be able to measure distance from the round trip delay.

The clue is in the original picture. The asset label says "TMD01001". TMD stands for "Train Mass Detector". There's also a "Tube Lines" label just above the barcode so it's been there several years.

There still seems to be some debate over what it triggers, but I agree with the last post that it's most likely to keep the track-to-train CCTV on while the train is in the station. There are lots of them around, certainly on the Northern Line.

There are a number of different types and it probably depends on whichever company won the OPO contract. I believe the contracts were awarded on a per-line basis so you may expect different styles of TMD for different lines.
 

Mawkie

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These are indeed Train Mass Detectors. These are used by the Station Management System to inhibit some announcements when trains are in the platform, or at other locations to start Mind the Gap announcements if required.

They work using ultrasound (85kHz) beyond the range of human hearing. To determine distance, the sound is emitted in pulses. Knowing how long the pulse takes to come back determines distance - and the units are configured to specific detection ranges.

As each pulse goes off, the frequency will rise through the range of human hearing into ultrasound, so you will be hearing the pulses go out. The beam is very directional so you do need to be right in front of it to hear.
 

bakerstreet

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These are indeed Train Mass Detectors. These are used by the Station Management System to inhibit some announcements when trains are in the platform, or at other locations to start Mind the Gap announcements if required.

They work using ultrasound (85kHz) beyond the range of human hearing. To determine distance, the sound is emitted in pulses. Knowing how long the pulse takes to come back determines distance - and the units are configured to specific detection ranges.

As each pulse goes off, the frequency will rise through the range of human hearing into ultrasound, so you will be hearing the pulses go out. The beam is very directional so you do need to be right in front of it to hear.
Thanks for this detailed answer - much appreciated

I must say I’m surprised at how this is done. Considering how advanced signalling, GPS, location services etc are today, this does seem a rather old fashioned way of identifying when a train is at a particular location to trigger / inhibit announcements etc.

Fascinating, thanks.
 

rebmcr

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Thanks for this detailed answer - much appreciated

I must say I’m surprised at how this is done. Considering how advanced signalling, GPS, location services etc are today, this does seem a rather old fashioned way of identifying when a train is at a particular location to trigger / inhibit announcements etc.

Fascinating, thanks.
Due to various insurmountable physical laws, methods of putting the locations of objects into computer systems are inherently inaccurate and/or imprecise, and often unreliable (camera lens dirt, fallen leaves blocking vents or circuits, etc). This still applies to modern, advanced systems.

Simply detecting the presence/absence of those objects is still subject to the same laws, but has fewer dimensions* and is therefore much easier to achieve — especially with 1990s technology as used on the Northern Line.

* e.g. this LU device has a simple on/off signal, and always deals with 'right now'. The inaccuracies to overcome apply to just one variable. GPS on the other hand tracks four variables (lat, long, altitude, and time), which it infers by triangulating signals from five or more sources. The inaccuracies multiply for each, making it about twenty times more difficult to be accurate. Smart programming can compensate and reduce the difference down from 20×, but probably not all the way down to 1×, even with 2020s tech — this is why Tesla Motors are often suspected of exaggerating their claims of self-driving car capabilities.
 

Domh245

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GPS is rather non-functional underground!!

It is likely an older installation, and so at the time of installation, a 'dumb' sensor like this would likely be cheaper than an interface to the signalling systems, especially if it is only controlling a single announcement trigger
 

100andthirty

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Train mass detectors have never been used for the OPO CCTV track to train systems. They are 'on' all the time. The cab equipment shows usually shows images as soon as it receives a signal and turns off when the signal is lost. The track side equipment uses a radiating cable antenna usually mounted inder the platform nosing stones. The S stock system is slightly different with both on and off triggers sent to the train as well as the main microwave signal. All the track equipment for the S stock system is mounted just above rail height in the 6 foot.
 

Mojo

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Train mass detectors have never been used for the OPO CCTV track to train systems. They are 'on' all the time. The cab equipment shows usually shows images as soon as it receives a signal and turns off when the signal is lost. The track side equipment uses a radiating cable antenna usually mounted inder the platform nosing stones. The S stock system is slightly different with both on and off triggers sent to the train as well as the main microwave signal. All the track equipment for the S stock system is mounted just above rail height in the 6 foot.
And on the Central line you can see the leaky feeder in the four foot.

It is likely an older installation, and so at the time of installation, a 'dumb' sensor like this would likely be cheaper than an interface to the signalling systems, especially if it is only controlling a single announcement trigger
Would also add to this, that it would have been installed in the day of the conventional signalling system with track circuits etc., but with the knowledge in the not too distant future (of course now completed several years ago) that the line was going to be resignalled, so even if a decision was made to utilise signalling equipment it would have needed replacing in time anyway.
 
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