The History
Before I go into the principle of operation and how I’ve modelled them on the layout, I’ll go into the history of Axle Counters.
The first use of an Axle Counter for train detection was at Glasgow Queen Street in 1967 due to problems of insulating a steel bridge from the required track circuits. After this, Axle Counters were generally only installed where providing a Track Circuit was troublesome or impractical, although they were particularly favoured in Scotland. Axle Counters first becoming widespread in 1999 when Queen Street was converted to all Axle Counters, and since then they have become the go to train detection for most resignalling.
The public probably became aware of Axle Counters in 1991 when it was suspected (although never proved amongst several theories) that an unauthorised reset of an Axle Counter section within Seven Tunnel on the 7th December caused the signal protecting the section to clear with a train within it, meaning a Sprinter could enter the occupied section and subsequently collide with the rear end of an HST.
Currently, there are three types of Axle Counter used in the UK from two manufactures. There is the traditional bolted ‘H’ series AzLM type from Thales which is most common; the clamped ‘K’ series AzLM type from Thales and the clamped RS123 from Frauscher, which is quickly becoming the favoured type despite being very new to the UK market.
Axle Counters v Track Circuits
This question has been argued and debated many times.
The advantages of an Axle Counter over a Track Circuit is that they are unaffected by any sort of conditions that would cause a Track Circuit to fail, such as rain, heat, snow, flooding (up to a point obviously, but much more tolerant than a Track Circuit), rusty rails and poor ballast resistance. Also, there are no joints to maintain, meaning less maintenance. With clamped Axle Counters, such as the ‘K’ series AzLM or RS123 is that they can be easily modified by simply loosening the head and sliding it along the rail (and extending / shortening the cable) rather than having to cut in a new joint. Axle Counters allow any vehicle with a flange to be detected, whereas there are lots of rail vehicles that can’t be relied upon to operate Track Circuits. Then there is the simple fact that they are more reliable than Track Circuits, which was the reason that over Christmas 2021 all the Train Detection sections between Paddington and West Drayton was converted to Axle Counters.
Having just said that Axle Counters are much more reliable, the disadvantage is that this requires careful setting up and installation, otherwise they can give trouble. Axle Counters must also be carefully positioned to prevent a train stopping with a wheel directly over the head, as a train slightly rolling back and forth when stopping or starting can confuse the evaluator. Axle counters also require data links and more complex interlocking and control date (see next section). If you have ever done PTS course for the railway or done P-Way Training for Axle Counter areas, you are told to never wave anything metal near it as it confuses it, this is only partially true. In fact, it is only a shovel that could affect the operation of an Axle Counter, whereas Ballast Forks, Pickaxes etc. won’t (but still, DON’T wave anything metal near an axle counter!) Another disadvantage is the fact that an Axle Counter won’t detect anything leaving the rail (such as through a derailment or an RRV coming off at an access point), but this can be managed through other means.
The most common ‘disadvantage’ of Axle Counters that is put forward is that of Broken Rail Detection. It is true that an Axle Counter will not detect a broken rail, however it is NOT TRUE that a Track Circuit WILL detect a broken rail, in practice it takes a clean split in the rail by quite a few mm in dry conditions for a broken rail to cause a Track Circuit to fail. Track Circuits have never been relied upon to detect broken rails and were never designed to do so. For instance, the broken rail that caused the Hatfield Rail Disaster was not detected by the Track Circuit over it. However, if you still believe that this is an overwhelming problem, then the standards to say that the maintenance regime must ensure that broken rails are picked up through other means (although this should be the case already) if Axle Counters are to be installed.
The current standards on the provision of Axle Counters is that they are the mandated standard Train Detection to be used for resignalling projects, IF it is intended for E.T.C.S. to be commissioned within 10 years of the resignalling (as Axle Counters are the preferred Train Detection for E.T.C.S.), however, they are pretty much standard Train Detection for most re-signalling.
My personal view is that Axle Counters are generally superior and more robust compared to Track Circuits, but Track Circuits do still have their place on the Railway
Operation
Axle Counters, be they Thales AzlM or Frauscher RS123, all work on the same principle. The system consists of three core components, the Head:
The Electrical Junction Box, also known as the ‘Yellow Mushroom’ (the image being a newer Thales EAK30K type, although the traditional Thales EAK30H is seen behind):
And the Evaluator (which I don’t have a photo of but is a computer module within a lineside location or within the interlocking room, but you won’t see that as a member of the public).
The head consists of a pair of transmitters (Tx’s) on the inside of one of the running rails, and a pair of receivers (Rx’s) on the outside of the running rails. The use of a pair of Tx’s and Rx’s allows direction of a train to be determined. The transmitter emits an electro-magnetic field around the rail head as below (the red and blue lines):
When the flange of a rail wheel passes over the head, it disturbs the electrical field, as below, causing a change in the resistance detected in the Receiver:
This change in resistance is passed to the electrical junction box (which also generates the field), which analysis this change and passes the information onto the evaluator, as (in basic terms) a wheel count (including direction), miscount or fault. The evaluator uses the information to count the number of axles into a section and then count the number of axles out. If the resulting value is anything but ‘0’, then the evaluator tells the interlocking the section is occupied and if it is ‘0’, then the interlocking is told the section is clear.
From an interlocking point of view, if everything is going well and working normally, axle counters and track circuits work identically, sections become occupied and then clear as trains pass through them. It is when things go wrong that they are very different. With a track circuit, the signaller doesn’t have to do anything before normal signalling can resume once the fault is fixed. However, axle counter sections require the signaller to reset / restore the section before normal signalling can resume once the fault is fixed.
The reset / restore procedure
There are four types of reset:
- Co-operative with Aspect Restriction = This is where the signaller and a technician lineside need to both carry out actions to reset the section, this is banned in new installations, but can be found in older installations.
- Conditional with Aspect Restriction = This is where the section will only reset if the last ‘count’ was out of the section (i.e., it is most likely the train has left).
- Unconditional with Aspect Restriction = This is used when Engineering Possession & Special Train Reminders are used and will reset the section whatever its state is.
- Preparatory = This is similar to conditional, but requires a train to actually prove the section is clear before the reset is successful and doesn’t apply aspect restriction
Almost all installation nowadays are Conditional / Unconditional. This is the reset that I’ve modelled on Collingwood, although as I don’t actually count train axles, then I’m really only modelling the signallers process, not the technical aspects.
xle Counter sections are portrayed differently to Track Circuits on a VDU workstation. They are identified by a (X) suffix and have a red roundel next the section I.D.
***In reality this roundel would act as the ‘button’ and indication, but on Collingwood I’ve made the track ID the button simply as I can’t have an indication acting simultaneously as a button due to the limitations in JMRI***
The actual reset procedure is generally carried out within the evaluator rather than in the Interlocking or Control System. Most resets are carried out after an Axle Counter Section fails to clear once a train as fully left it, therefore it is a pre-requisite that the section shows occupied with no routes set over it before a reset can take place. Obviously in the case of the a failure during the passing of a train, any route locking applied to that section and beyond will remain as the interlocking won’t have ‘seen’ the train clear that section, meaning that this type of failure can be very problematic, particularly at junctions. A reset can’t be performed if the section is disturbed (i.e. there is a fault somewhere within the head, junction box or evaluator), and no more than 2 resets can be carried on a section per day, this to prevent a more serious issue being masked by simply resetting the section. On Collingwood, my Axle Counter ‘failure’ is done manually by the signaller via a ‘Miscount’ button on the fault panel (just like the Point and Signal Lamp Failures shown in previous posts). So in the case of a train leaving, say, Platform 3 and ‘XP’ section miscounts, the train will pass the section, but the section will remain occupied, with the route held, even if the route is cancelled by the signaller:
Before the section is reset, it is procedural that the signaller applies an Engineering Reminder on the protecting signal (this isn’t enforced by the interlocking, just by the rule book):
To reset the section (which restores the ‘count’ of the section to ‘0’ and effectively makes the section go clear), the signaller on Collingwood presses the ‘A/C Reset’ button at the button of the screen and then selects the section to be reset.
***In reality, the signaller would simply press on the red roundel on the screen***.
This causes the red roundel to flash solid red whilst the evaluate resets it’s count to ‘0’, which can take a few seconds (on Collingwood, this is simulated by adding in a timing delay before the reset shows as successful):
Once the evaluator has successfully been reset, the section clears. However, this causes a problem in that the signaller, nor the evaluator, knows for certain that the section is physically clear (the failure of the section to clear might be because the whole train hasn’t left the section). This could mean that the signal could be cleared with a train in the section (exactly what was suggested had happened in the Seven Tunnel collision). Therefore, to prevent the signal clearing when the section goes occupied after a reset, something called ‘Aspect Restriction’ is applied to the protecting signal.
Aspect Restriction is where the signal is held at red, despite the route being clear in the electronic eyes of the interlocking, until it can be proven that the reset section is physically clear. This is done by talking a train past the signal at red at caution (a speed from which the driver can stop within their sighting distance), and if the train successfully pops out the other end without the section failing again, it can be taken that the section is actually clear (if it isn’t the train driver will stop and phone the signaller).
Whilst Aspect Restriction is in place, a dark red ‘Indication of Restricted Route’ background is shown around the signal to which Aspect Restriction is applied and section that is being reset (to tell the signaller why the signal is not clearing):
Once a train has passed through the section and the section re-clears, the Aspect Restriction is removed automatically and the IRR is no longer displayed. The signaller can then remove the engineering reminder and set routes normally.
What you see above is the routine for plain line sections, however it gets very complicated for sections involving points, dependent on where the Axle Counter heads are positioned on the ‘heel’ route at a turnout.
If the head is placed within 10m of the clearance point of the turnout, then the Aspect Restriction can be removed by a single train running along either route of the turnout. This is because the driver will be able to visually inspect the other route from the one they are taking up to 10m past the clearance point, anything further than that, it is agreed that the driver won’t be able to see, so won’t be able to confirm whether the portion of the section which they are not running along is clear.
So, if the head is placed more than 10m from the clearance point, then Aspect Restriction is applied separately through both routes of the turnout so that two trains, one along each route, would be required to fully remove Aspect Restriction from all routes. Instead, Aspect Restriction would only be removed for the portion of the section that the ‘sweep’ train runs along.
This can be both an advantage and a disadvantage, as it does require complex data and can mean a specially routed train if one route is rarely used, however, it does mean that you could use on route normally despite one part of the section still being in a ‘failed’ state whereas a track circuit failure would mean neither could be used unless the whole section is cleared.
EPRs & STRs
Of course there are times when multiple axle counters may fail and require re-set during known, pre-planned scenarios, such as possessions / engineering works, so to ease the workload on the signaller and to reduce the amount of time that it would restore the railway to its day to day running, extra controls can be implemented.
The most common is the ‘Engineering Possession Reminder’ (EPR). This is a control that the signaller can apply to multiple axle counter sections with an engineering possession which are known to be disturbed during the works. The control makes the section effectively ignore any mis-counts during the time it is applied and when it is removed the section is reset to ‘0’ without the need for Aspect Restriction. The ‘Line Clear’ verification being carried out separately as part of the possession handback process. Although, if one or both of the axle counter heads, junction box or cabling is damaged as part of the works, then the section will remain failed and show occupied even after the EPR is removed and will have to go through the normal re-set and Aspect Restriction process.
During the time an EPR is applied, a white background to the section to which is applied is shown (and flashes whilst it is applied and removed):
An unusual control is that of the ‘Special Train Reminder’ or STR control. This serves the same purpose as the EPR, however it is intended to be used when a train is to run through an Axle Counter and is known to not be reliably detected by the heads and cause a miscount. For instance a piece of on-track plant that may not have a proper flange profile, or where a train is known to have a damaged flange or wheel (for instance being towed for repair after a derailment). The signaller can apply the control ahead of the train arriving and then doesn’t need to reset all the sections after the train has passed. It is an usual control, simply as there aren’t many ‘Special Trains’ on the network, I’ve modelled it at Collingwood simply as the model is based so close to Eastleigh, a major repair and infrastructure maintenance facility, that it is likely that ‘Special Trains’ would run through the layout.
During the time an STR is applied, a bright green background is shown (again, flashing whilst being applied and removed):
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