Track circuits explained

[whole diary]

[grahame]

Track circuits explained ... by Network Rail

What are track circuits? How do they keep the railway safe? And why do they sometimes delay journeys?

A track circuit forms part of the broader signalling system, which comprises many parts to enable trains to move safely around the network.

A track circuit itself is an electrical system that detects the absence of a train on a section of track. This information then helps the signalling system know if it’s safe for another train to proceed.

etc

Good article (I think) providing just the sort of FAQ^▸ (Frequently Asked Questions) data we should be carrying on the Coffee Shop

[bobm]

I wonder how much of the railway operates with track circuits and how much now uses axle counters?

[johnneyw]

Track circuits explained ... by Network Rail

What are track circuits? How do they keep the railway safe? And why do they sometimes delay journeys?

A track circuit forms part of the broader signalling system, which comprises many parts to enable trains to move safely around the network.

A track circuit itself is an electrical system that detects the absence of a train on a section of track. This information then helps the signalling system know if it’s safe for another train to proceed.

etc

Good article (I think) providing just the sort of FAQ^▸ (Frequently Asked Questions) data we should be carrying on the Coffee Shop

Agreed and helps inform me of the very basics of a complex but personally, interesting subject.

[MVR S&T]

Further reading on track circuits at:

http://www.irse.org/minorrailways/publicdocuments/TC01%20v1-0%20DC%20Track%20Circuits.pdf

Page six has the basic circuit.

And they scale down to 7.25 inch too:

http://www.moorsvalleyrailway.co.uk/pages/signalling

[rower40]

I wonder how much of the railway operates with track circuits and how much now uses axle counters?

For the bits of the GW^▸ (Great Western) controlled from TVSC^» (Thames Valley Signalling Centre - about) at Didcot:
Paddington to Airport Junction, and Airport Junction to Heathrow: Track Circuits
Airport Junction to ... everywhere else: Axle Counters.
Except the B&H^▸ (Berks and Hants - railway line from Reading to Taunton via Westbury) (Berks and Hants) from Hungerford to Westbury (the non-electrified bit) still uses Track Circuits.
And a few track circuits still exist on fringes to other signalling centres, and depots/yards etc.

[Bmblbzzz]

I spy Filton Bank!

[paul7575]

If I understand it correctly though, as far as the overall signalling system is concerned the Axle Counter train detection is logically used in the same way as Track Circuit train detection?

The sections of railway with Axle Counters are still operated under track circuit block regulations, so they don’t in themselves make a massive operating change?

Paul

[rower40]

If I understand it correctly though, as far as the overall signalling system is concerned the Axle Counter train detection is logically used in the same way as Track Circuit train detection?

The sections of railway with Axle Counters are still operated under track circuit block regulations, so they don’t in themselves make a massive operating change?

Paul

Exactly so ... right up to the moment that a count of axles goes wrong, and the counter has to be reset.  Then there's a whole host of procedures to be followed, which have changed and evolved over the years.  Many of which have to be implemented in the interlocking logic and the control system.  As a technician-initiated reset is no longer allowed, the signaller has to reset the counter, then allow a "sweep train" through the section to prove that it's now clear.  The Sweep train is authorised past the entry signal at danger.

More complex yet if the axle counter section contains pointwork.  Because then the sweep train may have only proved that PART of the axle counter section is free from obstruction.

[bobm]

What are the advantages with axle counters?  Are they easier to maintain than track circuits?

[bradshaw]

Found this just now
https://www.arema.org/files/library/2010_Conference_Proceedings/Axle_Counters_vs_Track_Circuits-Safety_in_Track_Vacancy_Detection_and_Broken_Rail_Detection.pdf

As I see it the axle counter is a digital device whilst track circuits rely on a voltage drop which could vary for a number of reasons. One example was the introduction of Sprinter Cl 15x on the line between Castle Cary and Yeovil Pen Mill. While the old dmus operated the track circuits properly the 15x Sprinters would not always operate them leading to them disappearing from ‘view’. Additional electronics were needed to get a reliable operation.

[SandTEngineer]

Now, where are the S&T^▸ (Signalling and Telegraph) when you need them........oh.......

The are many types of track circuits, varying from very simple DC^▸ (Direct Current) operated ones to very complex Coded Digital ones.  Each type has it plusses and minuses (too complex to explain here).  There is one thing to be clear on though, and that is they are not guaranteed to detect a broken rail.  This is often quoted as being their advantage over axle counters.  The rail bonding rules are very complex and generally these days there is a huge risk from ending up with parallel bonding, in particular in electrified areas.  Track Circuits also have a high maintenance requirement (rail bonding, rail chair insulations, insulated rail joints etc.).  Hence the move towards axle counters.

[Reginald25]

Now, where are the S&T^▸ (Signalling and Telegraph) when you need them........oh.......

The are many types of track circuits, varying from very simple DC^▸ (Direct Current) operated ones to very complex Coded Digital ones.  Each type has it plusses and minuses (too complex to explain here).  There is one thing to be clear on though, and that is they are not guaranteed to detect a broken rail.  This is often quoted as being their advantage over axle counters.  The rail bonding rules are very complex and generally these days there is a huge risk from ending up with parallel bonding, in particular in electrified areas.  Track Circuits also have a high maintenance requirement (rail bonding, rail chair insulations, insulated rail joints etc.).  Hence the move towards axle counters.

I find the coded AC Track circuits fascinating, they work in a completely different way (I think!) to the DC relay based ones that are usually described in general information documents. They all seem to work so credit is due to the technicians.