Technical Questions regarding points mentioned in Sectional Appendices!

[Chrius56000]

. . .A quickie one!

. . .I found this mentioned in the London and North Western (North) Sectional Appendix, but I assume all seven of them contain the same or similar instructions!

. . .What is the reason for the prohibition on the use of Track Circuit Operating Clips where Axle Counters are in use?

. . .Do modern (second generation and after) DMU and EMU stock still carry Track Circuit Operating Clips? – when I used train in the late 1970s to the mid 1980s before the second generation DMUs came in, there was nearly always a T.C. clip on wooden blocks fixed to a bulkhead with a notice nearby – "Protection Of Line – Use Track Circuit Operating Clips FIRST!"

 

[godfreycomplex]

- Because they won't occupy the track section with axle counters, therefore they won't replace the signal to danger. So there's little point wasting time putting them down in an emergency when traincrew could be using their time more effectively

- Yes they do

 

[hexagon789]

. . .A quickie one!

. . .I found this mentioned in the London and North Western (North) Sectional Appendix, but I assume all seven of them contain the same or similar instructions!

. . .What is the reason for the prohibition on the use of Track Circuit Operating Clips where Axle Counters are in use?

. . .Do modern (second generation and after) DMU and EMU stock still carry Track Circuit Operating Clips? – when I used train in the late 1970s to the mid 1980s before the second generation DMUs came in, there was nearly always a T.C. clip on wooden blocks fixed to a bulkhead with a notice nearby – "Protection Of Line – Use Track Circuit Operating Clips FIRST!"

They do carry them.

I assume the prohibition is more because there would be little point, in an axle counter area TC clips won't do very much!

 

[pdeaves]

Applying track circuit operating clips won't cause any harm, but they won't do any good either.

== Doublepost prevention - post automatically merged: 1 Mar 2026 ==

For clarity for those who may not already know:
Track circuits use the running rails as part of the train detection system. Connecting the rails together (via train axle or operating clip) causes the system to detect a train [1].
Axle counters do not use the running rails but external equipment next to the rail. Such equipment doesn't really care what's on the rails, so long as the number of axles entering the section equals the number leaving/entering the next section.

[1] strictly, the system detects the absence of a train, according to a signalling designer I once knew, and stops detecting 'nothing' when a train is present.

 

[edwin_m]

[1] strictly, the system detects the absence of a train, according to a signalling designer I once knew, and stops detecting 'nothing' when a train is present.

Basically a voltage is put between the rails at one end of the track circuit and detected at the other end. If there is a pair of wheels or a track circuit clip in between then the voltage is shorted out and not detected. More modern track circuits follow the same principle but vary the voltage at multiple frequencies to reduce the risk of currents in the track causing a failure to detect.

 

[dosxuk]

Applying track circuit operating clips won't cause any harm, but they won't do any good either.

There is a potential for harm - in the aftermath of an incident, there would be no sign that they are present if they have been left attached to the track, and would lie in wait for the first train to come along.

 

[Skeletor]

There is a potential for harm - in the aftermath of an incident, there would be no sign that they are present if they have been left attached to the track, and would lie in wait for the first train to come along.

The weight of the train would annihilate a T-coc but agree you’d hope it wasn’t left attached.

 

[QueensCurve]

My question is what alternative protection could be provided and be effective in axle counter areas?

 

[NSEWonderer]

We still are expected to put them down over here on the Wessex sidem Feltham area resignalling saw axel counters added but several routes still use track circuit so I assume its to keep everything in line.

 

[High Dyke]

My question is what alternative protection could be provided and be effective in axle counter areas?

From my experience, a train occupying the axle counter section is effective enough. However, there are limitations...eg, you wouldn’t know if the whole train had derailed, unless it's destroyed the axle counter heads. Nor does it indicate a broken rail / bond wire.

 

[Lewisham2221]

My question is what alternative protection could be provided and be effective in axle counter areas?

You (in theory, at least) wouldn't need alternative protection. The train wouldn't have left the axle counter section, so the section would still show as occupied.

 

[edwin_m]

My question is what alternative protection could be provided and be effective in axle counter areas?

I assume this question relates to a derailed train fouling an adjacent line, which is the situation where the track circuit clip is used.

All trains now have GSM-R radios with an emergency stop that goes to all other trains in a certain area. That would seem to be far more effective than a track circuit clip, quicker to apply and doesn't rely on an approaching train seeing a red signal. It would have problems if the radio reception was poor or if damage to the train made the radio inoperative.

 

[PupCuff]

I might be having a dopey moment, but where does the sectional appendix prohibit the use of track circuit operating clips in axle counter areas?

Are you sure you aren’t getting mixed up with track circuit operating devices?

 

[LCC106]

I’ve never heard of the prohibition of use in areas where axle counters exist! Thanks for this, must do some digging!

 

[DeparturesBeck]

I believe the OP is confusing track circuit operating clips (TCOC) with track circuit operating devices, TCOC are still used as standard for emergency protection.

 

[edwin_m]

I believe the OP is confusing track circuit operating clips (TCOC) with track circuit operating devices, TCOC are still used as standard for emergency protection.

Neither of them will work in an axle counter area!

 

[DeparturesBeck]

Neither of them will work in an axle counter area!

Correct however that is not what the OP is asking, they asked "what is the reason for the prohibition on the use of Track Circuit Operating Clips where Axle Counters are in use".

The answer is that the sectional appendix does not prohibit the use of track circuit operating clips in axle counter areas as the OP believes, it prohibits the use of track circuit operating devices.

 

[zwk500]

The answer is that the sectional appendix does not prohibit the use of track circuit operating clips in axle counter areas as the OP believes, it prohibits the use of track circuit operating devices.

Are these two distinct things or are Clips a subset of Devices?

 

[Skeletor]

Are these two distinct things or are Clips a subset of Devices?

They are distinct.

A T-coc clips onto the top of either rail and shorts out the track circuit. It’s used in emergencies as a quick and efficient method of occupying the track and puts the protecting signal to danger.

A T-Cod is attached onto the bottom of either rail and you actually use a key to secure it on. You can use a T-cod as a method of additional protection for a line blockage and this would be agreed with the signaller along with your protecting signal(s).

 

[edwin_m]

The answer is that the sectional appendix does not prohibit the use of track circuit operating clips in axle counter areas as the OP believes, it prohibits the use of track circuit operating devices.

While I was looking this up I noticed that track circuit operating clips are mandated following the use of an emergency sander if it's not possible to contact the signaller immediately, or if the signaller is contacted but can't see the train occupying a track circuit. This is because of the risk that the sand discharge is insulating the rails from the wheels and thus making the train invisible to the signalling.