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Track circuits are a pain - but any replacement has to provide the same level of safety when things go wrong
Engineers and operators can't wait to get rid of track circuits. Civil engineers don't like them because they have meant inserting insulated joints into nice continuous steel rails. Jointless track circuits get over this objection, but then you have impedance bonds and other kit between the rails which gets damaged during tamping, or fails, of course.
Traction engineers really hate them because you have to spend millions proving that any credible fault with your three phase drive won't confuse an antique track circuit on a wet Wednesday when the moon is in Pisces. Operators hate them because they fail-safe and stop the railway.
So, Railtrack adopted a policy of replacing electric track circuits with axle counters. Until of course ETCS Level 3 gets rid of all trackside kit – allegedly – one day – not soon.
On the West Coast Route Modernisation, axle counters are already being used for train detection on the recent Euston resignalling and were scheduled to be installed on the North Staffs scheme. Note the use of the imperfect tense. As we will see, failure to think through this policy has dealt yet another blow to the ill fated project.
First, a tiny technical primer. Track circuits are very simple. You have an electrical current in one rail that holds a relay closed. This relay completes a circuit to the signal interlocking. So long as current flows in this circuit, the interlocking gets an electric signal denoting that the particular track circuit section is unoccupied.
If a train enters the circuit, its wheels and axles act as a short -circuit, the relay loses its current and opens, breaking the circuit to the interlocking. Without the signal from the relay, the interlocking assumes that the section is occupied and turns the protecting signal to danger.
To see how they work, start with an empty section.
A device on the rail at the start of the section counts the number of axles as a train passes and transmits this number to a comparator. The comparator deduces that since 40 axles, say, have gone into the section it must be occupied and generates an electrical ‘section occupied' signal for the interlocking.
At the other end of the section is another axle counter which counts the axles as the train leaves and tells the comparator the number. If the answer is 40, the comparator deduces that the train that entered the section has now exited and has not parted and left a vehicle in section. ergo a ‘section clear' signal is sent to the interlocking.
Axle counters have been used for years on long single track sections abroad. They are invaluable in wet locations like the Severn Tunnel where track circuits are unreliable. There use on European multi-track railways is growing.
So goodbye track circuit, welcome axle counter? Err, not quite, because Railtrack, but not my experienced old railway signalling chums whose collective wisdom Railtrack studiously ignores, has overlooked some vital safety benefits which come free with track circuits.
And operational benefits too. For a start, a track circuit is a yes/no device which provides a positive indication that a train is in section. Forget Pacers running on thick mulch for the time being.
Now suppose the signalling system goes down. When it comes back on again, each track circuit immediately generates clear or occupied indications to the interlocking.
In a comparable situation, each axle counter comparator may remember its status before the failure. But suppose there was some hand signalled working during the hiatus. Until things start moving the comparator has no idea whether section is occupied .
In other words, track circuits initialise interlockings automatically, while axle counters don't. To get the system working you have to run each train slowly into the next section before the signalling system knows where it is.
Think that's unlikely? When someone cut through a cable at Euston at the beginning of May, shutting off power in the process, the batteries of the axle counters went flat leaving the interlockings blind and delaying recovery somewhat.
So axle counters may not be the operator's dream after all when it comes to the modern four track high speed, high intensity railway, rather than the single line up country. And that's before you get onto safety.
Railtrack's new possessions policy is based on the track crew providing its own protection through the signalling system using a Track circuit Operating Device (TCOD). So what happens if a section of track is protected with axle counters?
Now consider the Hatfield derailment. As a brief digression, I'm still trying to bottom various issues on track circuiting and bonding in the section where the rail broke.
It is reported that there were two examples of ‘showing clear when occupied' (SCWO) in a period of 90 minutes before the accident. Now SCWO is usually associated with a two car diesel multiple unit or Pacer at leaf-fall time or on rusty rails. It is very hard for a track circuit to lose a loco and nine vehicles on shiny well used rail. Informed Sources suggest that at least a dozen trains passed over the break before the catastrophic failure of the high rail.
But, once the train left the track and started ripping up the adjacent lines, track circuits were interrupted and protecting signals turned to red. Had axle counters been fitted, the signalling system would have assumed all was normal, with the possibility of other trains running into the wreckage.
But supposing, an accident left a train foul of adjacent running lines without damaging the other rails? If, dear reader, you are on a DOO train which has such an accident and incapacitates the driver, do not emulate the railway children and run up the track waving your arms.
Find the track clips – a length of wire with a monster bulldog clip at each end – and put a clip on each of the rails of the fouled track. This will short out the track circuit and set the protecting signal to danger. Richard Hope, the doyen of the railway press, had the presence of mind to do this at the Wembley accident.
Replacing track circuits with axle counters on a busy high speed multi-track route like the West Coast Main Line, would mean a less safe railway unless there is some alternative to track clips. The obvious alternative, although still not as effective in my book, is to provide driver and guard with instantaneous emergency radio communication with the signaller.
This will be relatively simple with the digital GSM-R radio system which is coming in as part of the European Train Control System Level 2 on the WCML in 2005. With GSM-R, the guard could have a mobile handset with a red emergency button, which he could use to alert the signalman who would then stop all trains. But, we haven't got GSM-R yet and Railtrack was planning to use axle counters on the North Staffs resignalling with no compensation for the reduced level of safety.
So now, according to Informed Sources, axle circuits have been rejected by the safety authorities and the scheme is having to be redesigned for conventional track circuits which are quite different when it comes to things like equipment location and power supplies. Interestingly, there was an editorial in the newsletter of the Institution of Railway Signal Engineers (the only institution which will have me as a member) on the issue of track circuits versus axle counters in January this year. So Railtrack should have seen it coming.