Insulated Block Joints - are they on one rail or both?

[_Henry_]

Hello,

I am a bit confused about IBJs. I thought for a track circuit to work, you must have IBJs at either end of the track circuit and on both rails. Surely, if you don't do this then the track circuit would always be shorted? However, I have heard that on overhead electrification lines, you only put one IBJ in and it can not be in the 'common rail'. Is this right?

In my head, I can't see how this stops a short circuit as the electricity can still flow through the other rail.

 

[norbitonflyer]

As I understand it, both rails carry the current, and the circuit is completed by a relay connected between them, or by a train short-circuiting the circuit so the relay detects a loss of current. The track can be separated into different sections by putting an IBJ in either rail.

It could be both, but on electrified lines the rails also act as an earth return. Putting IBJs in makes them less effective in this role (!), so an IBJ is used in one rail only. The other is thus a continuous earth potential rail, both for traction current and track circuit purposes.

(At least, that's how I understand it works)

 

[Annetts key]

"Single Rail" Track Circuits


In the above diagram, I am showing two track circuits. The rails of the track are shown as thicker black coloured lines. The bottom rail has no Insulated Rail Joints (IRJ or IBJ). The top rail is shown with three IRJs, each shown as a vertical blue colour line.

Below the bottom rail, I have shown symbols for a cell (battery) to represent the track circuit feed sets. And also shown symbols for the track circuit relays.

I have shown the interconnecting cables in thin red coloured lines for the cables (wires) carrying the positive polarity current. And in a dark grey colour for the interconnecting cables carrying the negative polarity current.

I have also added + (for positive) and - (for negative) symbols at various places to again indicate the polarity.

Please note that as there are no intended electrical connections to earth (ground), each feed set electrically is independent to any other.


With no trains present, for track circuit AA, the positive polarity current flows from the feed set shown in the middle to the top rail, then along it to the left, then down to the positive connection on the relay. The negative current flows from the negative connection on the relay along the bottom rail back to the negative of the feed set. As a result, the relay is energised (or "up").

The same happens for the adjacent track circuit, here called AB, except that now the top rail is negative and the bottom rail is positive.

On the top rail, the left hand IRJ stops track circuit AA from affecting or being affected by anything (including trains) to the left of the IRJ. The middle IRJ stops track circuit AA from affecting or being affected by track circuit AB or from trains to the right of this IRJ. Similarly, track circuit AB is not affected by track circuit AA or any trains to the left of the middle IRJ. It's similar again for the right hand IRJ.

So, you may at this point have a question. Something along the lines of "but on the bottom rail, in the middle you show both positive and negative symbols next to one another, how can that be correct without there being an IRJ to keep them separated?"

Ahh, well, this is where you have to remember that each feed set is electrically independent from one another. The polarity shown is relative to that of the feed set ONLY. So in the real world, the whole length of the bottom rail is negative for track circuit AA (feed set and relay) no matter where you are. But for track circuit AB (feed set and relay), the whole length of the bottom rail is positive.

Now when a train moves on to track circuit AA, the wheels and axle of each wheel set will short the top rail to the bottom rail. This results in an alternative route for the electrical current from the feed set for AA track circuit. As a result, very little or no current gets to the relay for this track circuit, so relay AA de-energises (drops). Note that this has absolutely no effect on track circuit AB.

 

[GRALISTAIR]

This forum is an absolutely fantastic source of information. Thanks to the OP for posting and the respondents for their answers.

 

[hwl]

Hello,

I am a bit confused about IBJs. I thought for a track circuit to work, you must have IBJs at either end of the track circuit and on both rails. Surely, if you don't do this then the track circuit would always be shorted? However, I have heard that on overhead electrification lines, you only put one IBJ in and it can not be in the 'common rail'. Is this right?

In my head, I can't see how this stops a short circuit as the electricity can still flow through the other rail.

It depends on whether you have:
a) DC track circuits (the example above) - OHLE areas - single rail possible
b) low frequency AC track circuits (e.g. 50Hz) - 3rd rail areas - double rail IBJ (and impedance bond to filter AC but not DC across the IBJs) normally needed
c) low frequency AC track circuits (e.g. 33or 50Hz) - LU 4th rail areas - single rail IBJ possible
d) high (tuned audio frequency) - newer installation - Zero IBJs as the frequency for each circuit is different

 

[Skeletor]

Hello,

I am a bit confused about IBJs. I thought for a track circuit to work, you must have IBJs at either end of the track circuit and on both rails. Surely, if you don't do this then the track circuit would always be shorted? However, I have heard that on overhead electrification lines, you only put one IBJ in and it can not be in the 'common rail'. Is this right?

In my head, I can't see how this stops a short circuit as the electricity can still flow through the other rail.

The very basic premise of electric current flow is that you need a complete loop in a circuit and a potential difference between two points. That potential difference is the feed set itself.

Follow both of the feed outputs from the above diagram that Annetts has posted, they both return to the negative side of the feed set as there is no path through the IRJ.

 

[Zomboid]

"Single Rail" Track Circuits
View attachment 180746

In the above diagram, I am showing two track circuits. The rails of the track are shown as thicker black coloured lines. The bottom rail has no Insulated Rail Joints (IRJ or IBJ). The top rail is shown with three IRJs, each shown as a vertical blue colour line.

Below the bottom rail, I have shown symbols for a cell (battery) to represent the track circuit feed sets. And also shown symbols for the track circuit relays.

I have shown the interconnecting cables in thin red coloured lines for the cables (wires) carrying the positive polarity current. And in a dark grey colour for the interconnecting cables carrying the negative polarity current.

I have also added + (for positive) and - (for negative) symbols at various places to again indicate the polarity.

Please note that as there are no intended electrical connections to earth (ground), each feed set electrically is independent to any other.


With no trains present, for track circuit AA, the positive polarity current flows from the feed set shown in the middle to the top rail, then along it to the left, then down to the positive connection on the relay. The negative current flows from the negative connection on the relay along the bottom rail back to the negative of the feed set. As a result, the relay is energised (or "up").

The same happens for the adjacent track circuit, here called AB, except that now the top rail is negative and the bottom rail is positive.

On the top rail, the left hand IRJ stops track circuit AA from affecting or being affected by anything (including trains) to the left of the IRJ. The middle IRJ stops track circuit AA from affecting or being affected by track circuit AB or from trains to the right of this IRJ. Similarly, track circuit AB is not affected by track circuit AA or any trains to the left of the middle IRJ. It's similar again for the right hand IRJ.

So, you may at this point have a question. Something along the lines of "but on the bottom rail, in the middle you show both positive and negative symbols next to one another, how can that be correct without there being an IRJ to keep them separated?"

Ahh, well, this is where you have to remember that each feed set is electrically independent from one another. The polarity shown is relative to that of the feed set ONLY. So in the real world, the whole length of the bottom rail is negative for track circuit AA (feed set and relay) no matter where you are. But for track circuit AB (feed set and relay), the whole length of the bottom rail is positive.

Now when a train moves on to track circuit AA, the wheels and axle of each wheel set will short the top rail to the bottom rail. This results in an alternative route for the electrical current from the feed set for AA track circuit. As a result, very little or no current gets to the relay for this track circuit, so relay AA de-energises (drops). Note that this has absolutely no effect on track circuit AB.

Is that the case in DC electrified areas where both rails are used for traction current and are insulated from earth? Looking at the impedance bonds it appears that both rails are (electrically) broken, but I don't know what goes on inside.

AC electrified areas usually only use one rail for traction current so that explanation would hold there.

Edit- ok, I just saw the "single rail" in the title... Which means it doesn't apply to double rail track circuit areas, such as DC lines...

 

[Annetts key]

Is that the case in DC electrified areas where both rails are used for traction current and are insulated from earth? Looking at the impedance bonds it appears that both rails are (electrically) broken, but I don't know what goes on inside.

AC electrified areas usually only use one rail for traction current so that explanation would hold there.

Edit- ok, I just saw the "single rail" in the title... Which means it doesn't apply to double rail track circuit areas, such as DC lines...

The original question was about overhead electrification lines.

I'm not the best person to ask about DC electrified lines, having never worked on a DC third rail area.

For lines where there is no electrification, with conventional track circuits, both rails would be provided with IRJs (IBJs) because that improves reliability of the track circuits (a failure of a single IRJ that separates two different track circuits doesn't result in a track circuit failure). These being classed as double rail track circuits.

By conventional track circuits I mean any of the various types of DC track circuit, Westinghouse / BR WR Quick Release (an AC track circuit that uses a DC relay) or the impulse track circuits.

Audio frequency AC track circuits are also double rail track circuits, but don't need IRJs (IBJs) to separate them. But do need IRJs if they butt up to other track circuit types. Some classes of these can be used in AC OHL areas with additional equipment.

 

[Ploughman]

Are Aster track circuits still in use?
And how did they work with no joints?

 

[Annetts key]

Are Aster track circuits still in use?
And how did they work with no joints?

As of two years ago, yes, there were still some ASTER "U" / SF15 type in use on part of Western Route. I don't know if they have been replaced. Elsewhere on Western, a rolling programme had seen many of the "U" / SF15 type replaced with the more modern TI21 / EBI type. This started before the electrification of the GWML. Combined with axle counters replacing track circuits on the area covered by the resignalling of Swindon and Bristol PSB areas, this resulted in all existing "U" / SF15 type being replaced on the MLN all the way to Bridgwater. And all being replaced between Patchway Junction and Swindon.

I don't know about elsewhere.

The way that ASTER "U" / SF15 type and TI21 / EBI type work is that they take advantage of the self inductance of a conductor, such as that of a rail. This is combined with capacitors and inductors in lineside units. Combined, filters are formed. These separate the different frequency audio frequencies used by this type of track circuit. Each separate section uses a different frequency compared to the adjacent sections.

Because the filters form an electrical separation at the audio frequencies, IRJs are not needed.

 

[MarkyT]

As of two years ago, yes, there were still some ASTER "U" / SF15 type in use on part of Western Route. I don't know if they have been replaced. Elsewhere on Western, a rolling programme had seen many of the "U" / SF15 type replaced with the more modern TI21 / EBI type. This started before the electrification of the GWML. Combined with axle counters replacing track circuits on the area covered by the resignalling of Swindon and Bristol PSB areas, this resulted in all existing "U" / SF15 type being replaced on the MLN all the way to Bridgwater. And all being replaced between Patchway Junction and Swindon.

I don't know about elsewhere.

The way that ASTER "U" / SF15 type and TI21 / EBI type work is that they take advantage of the self inductance of a conductor, such as that of a rail. This is combined with capacitors and inductors in lineside units. Combined, filters are formed. These separate the different frequency audio frequencies used by this type of track circuit. Each separate section uses a different frequency compared to the adjacent sections.

Because the filters form an electrical separation at the audio frequencies, IRJs are not needed.

Great explanation! The Aster company was French and created these earliest examples of jointless TCs after a decade of R&D in the 1960s. Their products were manufactured under license in the UK by ML Engineering of Plymouth, and the TI21 was MLs own development of the same concept, incorporating work from Ericsson of Sweden, which was the first jointless type to be used on electrified lines in the UK. TI21s were used extensively in many 1980s resignalling schemes such as Exeter and Westbury.

Their installation along the Dawlish sea wall and adjacent Exe and Teign estuary sections soon proved to be a major reliability problem and they were replaced in a number of phases by the largest installation of multiple axle counter sections in the UK at the time, after a short period using a temporary special long section Absolute Block style system that could be switched in by Exeter's panel supervisor during high seas. That required tail-light observation by staff specially posted at Teignmouth and Dawlish Warren. It wasn't a particular problem with TI21s at Dawlish. The salt spray dramatically changed the ballast resistance and would be a problem with any kind of track circuit with or without IBJs. The previous Absolute Block working between the individual signal boxes employed only a tiny handful of very short track circuits. It's remarkable that no one identified the potential problem of introducing continuous TCs through an area regularly exposed to large quantities of salt water.

Here's a training manual for Aster track circuits. https://dickthesignals.co.uk/onewebmedia/aster book.pdf

 

[Annetts key]

The Aster track circuits were replaced in the Chipping Sodbury Tunnel area by axle counters, also because of semi-regular flooding. I don't know when the first axle counters went in there, but as a trainee, I was dragged along to the on-site training course just after a new axle counter system was commissioned (one axle counter system for the up line and another axle counter system for the down line) in 1987.

 

[MarkyT]

The Aster track circuits were replaced in the Chipping Sodbury Tunnel area by axle counters, also because of semi-regular flooding. I don't know when the first axle counters went in there, but as a trainee, I was dragged along to the on-site training course just after a new axle counter system was commissioned (one axle counter system for the up line and another axle counter system for the down line) in 1987.

I finished my training scheme and took up my first drawing office position at Reading in 1986. Axle counters were becoming a big thing but were still considered expensive so application areas were very specific to places where track circuits couldn't be relied on to work reliably or where a single axle counter could replace a series of individual track circuits. As well as the sea wall, over the rest of the decade, they were installed in the Severn Tunnel and in a number of schemes for longer block sections on secondary lines. Westbury resignalling used one for the Castle Cary - Yeovil Pen Mill single line for example. I worked a weekend assisting testing on that project; that was the first time I'd seen the equipment in the flesh.

 

[Belperpete]

Is that the case in DC electrified areas where both rails are used for traction current and are insulated from earth? Looking at the impedance bonds it appears that both rails are (electrically) broken, but I don't know what goes on inside.

AC electrified areas usually only use one rail for traction current so that explanation would hold there.

Edit- ok, I just saw the "single rail" in the title... Which means it doesn't apply to double rail track circuit areas, such as DC lines...

Both double-rail and single-rail track-circuits are used in DC areas. While modern AC electrification schemes have tended to use single-rail track-circuits, older schemes used double-rail track-circuits in plain-line areas. Both single-rail and double-rail track-circuits can therefore be found in both AC and DC traction areas, and in non-electrified areas.

With double-rail track-circuits, each pair of insulated joints should ideally be exactly parallel. However, this is usually impossible to achieve in points and crossings (P&C), and in particular in crossovers. It is therefore common to use single-rail track-circuits in P&C, with one rail common to one or more track-circuits. In traction areas, the common rail is also used for the traction return current and so is termed the “traction rail”, while the other rail is termed the “signalling rail” or “track-circuit rail”. Despite its name, the traction rail doesn't just carry the traction return, but also the track-circuit return current.

With double-rail track-circuits, impedance bonds (a type of transformer) have to be provided, which allow the traction current to by-pass the insulated joints, while blocking the track-circuit frequency. These are not necessary with single-rail track-circuits, where the traction current flows through the common rail. However, because the traction returns through only one rail, this gives a higher impedance, which can limit the use of single-rail track-circuits. For example, in DC traction areas (which can have very high traction currents), single-rail track-circuits are not usually permitted in plain-line, only in P&C or for a relatively short distance beyond. In AC traction areas (which have higher voltages and therefore lower traction currents), single-rail track-circuits are more widely used, provided regular return or cross-bonding is provided to assist the traction return. However, in multi-track areas, the traction return bonding can provide unexpected paths for the track-circuit current to flow through. It has even been known for a track-circuit to still show clear with a whole rail removed, with the track-circuit return current continuing to flow via the traction bonding!

== Doublepost prevention - post automatically merged: 14 Jun 2025 ==

Great explanation! The Aster company was French and created these earliest examples of jointless TCs after a decade of R&D in the 1960s. Their products were manufactured under license in the UK by ML Engineering of Plymouth, and the TI21 was MLs own development of the same concept, incorporating work from Ericsson of Sweden, which was the first jointless type to be used on electrified lines in the UK. TI21s were used extensively in many 1980s resignalling schemes such as Exeter and Westbury.

IIRC, don't the TI21 incorporate a pulse feature, to help break-down rusty rail? This would not help in wet rail conditions.

Another type of jointless track-circuit was the GEC “reed” track-circuit. This used a metal reed tuned to a particular frequency. At one time "reeds" were quite widely used, for FDM remote-control systems, point detection in dual-traction areas, and jointless track-circuits.

The reed jointless track-circuits had a big plus in that you could overlap the track-circuits. It was therefore not necessary to provide a separate overlap track-circuit beyond a signal, you just extended the rear track-circuit to end not at the signal but at the end of the overlap. A train standing in the overlap would therefore be detected by both rear and forward track-circuits. This gave a significant saving on the number of track-circuits needed.

Although they had been quite widely used elsewhere without problems, a significant issue was found with the reed jointless track-circuits installed on the London Victoria resignalling scheme. If you had a series of short sections, a train accelerating from rest and drawing a high traction current could "suck" track-circuit current from a track-circuit in rear of the same frequency, sufficient to operate the relay of a track-circuit with a train standing on it. All the reed tracks on the Victoria scheme had to be quickly converted to jointed track-circuits - this involved cutting quite a lot of insulated joints into the CWR, much to the dismay of the PW! The problems were at first thought to be due to having to use a different type of tuning unit that wouldn't be fried by the DC traction return, however it was realised that although much less likely, the same problem could potentially also occur with standard tuning units.

 

[Annetts key]

IIRC, don't the TI21 incorporate a pulse feature, to help break-down rusty rail? This would not help in wet rail conditions.

TI21 continuously alternate between two different frequencies (in other words, the centre frequency is modulated) unless configured for ASTER U / SF15 compatibility mode in which case they stay on a single frequency.

Both ASTER U / SF15 and TI21 can sometimes "jump" very small air gaps. I know that if the rust on the rail head is too bad, an ASTER U / SF15 may fail to detect a train, as this has happened. I don't know about TI21.

Another type of jointless track-circuit was the GEC “reed” track-circuit. This used a metal reed tuned to a particular frequency. At one time "reeds" were quite widely used, for FDM remote-control systems, point detection in dual-traction areas, and jointless track-circuits.

Yes, vital FDM remote control is still used. Now made by Unipart IIRC.

 

[MarkyT]

Yes, vital FDM remote control is still used. Now made by Unipart IIRC.

On DC electrified lines of the Southern Region, extensive vital reed telemetry systems from the 1970s and 80 have been replaced by a digital system, beginning in the early 2000s. Sold by Siemens as Westplex, the new system was initially known by its American name, HD-link. It's difficult to find any mention of it specifically under either name on the current Siemens website so possibly superseded by newer products, or 'solutions' as they refer to endlessly.

There had been recurring, albeit rare, intermittent wrong side failures with reed in these applications that boffins had been unable to get to the bottom of and fully mitigate, making a strong safety case for the early renewal of the particular subsystem. The concerning incidents only occurred on DC lines, which already used a more limited set of reed frequencies than non-electrified installations to avoid mains harmonics, so there was a consensus the problem was definitely traction-related, and there wasn't the same urgency to replace reed elsewhere.

HD-link was an established North American Safetran product, brought into the UK market after Westinghouse became part of the Invensys conglomerate. In the USA, it was widely used over spread spectrum data radio links for block and aspect controls between interlockings, sometimes as an emergency replacement for tornado-ravaged pole routes. The product name echoes the H and D relays of lineside aspect circuits, the yellow (HR) and green (DR) relays. N. American and UK signalling designs share the same signal circuit naming conventions and it's thought the yellow and green circuit function letters derive from historic 'Home' and 'Distant' nomenclature.

SIL4 rated (the highest integrity level for safety critical systems), HD-link provides fully distributed telemetry, with no central processor node. It can be configured to connect a virtual circuit between any input and output on any remote 8-in, 8-out controller module of the system, which has a virtually unlimited address space. Each virtual circuit can also incorporate some simple logic and timer functions where desired. In the UK, Westinghouse employed a specialist partner to develop a custom comms module allowing the local LonWorks protocol messaging to be converted to standard IP ethernet packets and carried between clusters of modules via DSL (digital subscriber line) modems over the existing copper pairs previously used by the reed.

 

[TSG]

On DC electrified lines of the Southern Region, extensive vital reed telemetry systems from the 1970s and 80 have been replaced by a digital system, beginning in the early 2000s. Sold by Siemens as Westplex, the new system was initially known by its American name, HD-link. It's difficult to find any mention of it specifically under either name on the current Siemens website so possibly superseded by newer products, or 'solutions' as they refer to endlessly.

There had been recurring, albeit rare, intermittent wrong side failures with reed in these applications that boffins had been unable to get to the bottom of and fully mitigate, making a strong safety case for the early renewal of the particular subsystem. The concerning incidents only occurred on DC lines, which already used a more limited set of reed frequencies than non-electrified installations to avoid mains harmonics, so there was a consensus the problem was definitely traction-related, and there wasn't the same urgency to replace reed elsewhere.

HD-link was an established North American Safetran product, brought into the UK market after Westinghouse became part of the Invensys conglomerate. In the USA, it was widely used over spread spectrum data radio links for block and aspect controls between interlockings, sometimes as an emergency replacement for tornado-ravaged pole routes. The product name echoes the H and D relays of lineside aspect circuits, the yellow (HR) and green (DR) relays. N. American and UK signalling designs share the same signal circuit naming conventions and it's thought the yellow and green circuit function letters derive from historic 'Home' and 'Distant' nomenclature.

SIL4 rated (the highest integrity level for safety critical systems), HD-link provides fully distributed telemetry, with no central processor node. It can be configured to connect a virtual circuit between any input and output on any remote 8-in, 8-out controller module of the system, which has a virtually unlimited address space. Each virtual circuit can also incorporate some simple logic and timer functions where desired. In the UK, Westinghouse employed a specialist partner to develop a custom comms module allowing the local LonWorks protocol messaging to be converted to standard IP ethernet packets and carried between clusters of modules via DSL (digital subscriber line) modems over the existing copper pairs previously used by the reed.

Unfortunately Westplex is now obsolete and I'm not aware of a replacement. I suspect all vital reed on Southern is gone now, either through Westplex replacement or complete resignalling. Pity though, because Westplex may have given useful options for re-controlling areas with non-vital FDM

 

[MarkyT]

Unfortunately Westplex is now obsolete and I'm not aware of a replacement. I suspect all vital reed on Southern is gone now, either through Westplex replacement or complete resignalling. Pity though, because Westplex may have given useful options for re-controlling areas with non-vital FDM

The Wayside Communication System product line may have replaced it. There's a lot more functionality bundled in with vital telemetry, including office-field remote control links, track train radio for PTC, sophisticated alarms, diagnostics etc. I think it was a response to the needs of the US PTC programme where railroads had to roll out many new remote signalling installations quickly. Can't find any detailed data sheets though.

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Complicating matters is a developing standard for connecting interlocking situated 'anywhere' with remote lineside assets, known as EULYNX. This would allow easier interconnection of different manufacturers' systems and components. Some suppliers envisage selling interlocking as a service 'in the cloud' by this means, to which I respond yuck!. By definition, such a system must allow vital communication along the trackside however.