Axle counter: Wikis

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An axle counter detection point in the UK

An axle counter is a device on a railway that detects the passing of a train in lieu of the more common track circuit. A counting head (or 'detection point') is installed at each end of the section, and as each axle passes the head at the start of the section, a counter increments. A detection point comprises two independent sensors, therefore the device can detect the direction of a train by the order in which the sensors are passed. As the train passes a similar counting head at the end of the section, the counter decrements. If the net count is evaluated as zero, the section is presumed to be clear for a second train.

This is carried out by safety critical computers called 'evaluators' which are centrally located, with the detection points located at the required sites in the field. The detection points are either connected to the evaluator via dedicated copper cable or via a telecommunications transmission system. This allows the detection points to be located significant distances from the evaluator. This is useful when using centralised interlocking equipment but less so when signalling equipment is distributed at the lineside in equipment cabinets.

Contents

Advantages

Unlike track circuits, axle counters do not require insulated rail joints to be installed. This avoids breaking the continuity of long welded rails for insulated joints to be inserted. Such joints introduce a weak point in the rail, where a broken rail is more likely to occur. The cause of many track circuit failures can be traced to problems with insulated rail joints.

Axle counters require no bonding and less cabling in comparison to track circuits, and are therefore generally less expensive to install and maintain.

Axle counters do not suffer problems with railhead contamination, e.g. due to rust or compacted leaf residue, that can affect the correct operation of track circuits.

Axle counters are used in places such as wet tunnels (such as the Severn Tunnel), where ordinary track circuits are unreliable. Axle counters are also useful on steel structures which prevent the normal operation of track circuits. Axle counters are also useful on long sections where several intermediate track circuits may be saved.

Experience with axle counters in mainland Europe shows that they regularly achieve up to five times the reliability of track circuits carrying out the same function. This has immediate improvement in service reliability as track circuit failure is often the most significant cause of train delay. It also has safety benefits as it reduces the use of degraded modes of operation outside of the control of the signalling system due to failure.

A Frauscher axle counter sensor, for example, can be 8500m from the evaluation unit. [1]

Disadvantages

Axle counters may 'forget' how many axles are in a section for various reasons such as a power failure. A manual override is therefore necessary to reset the system. This manual override introduces the human element which may be unreliable. An accident occurred in the Severn Tunnel and is thought to be due to improper restoration of an axle counter. This, however, was not proven during the subsequent inquiry.

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Fail safety

Axle counters only provide intermittent positive indication of a rail vehicle as it passes a fixed location. If the counter unit fails or becomes disconnected, a train will pass undetected into a block that would otherwise be regarded as unoccupied. Track circuits provide continuous real time detection over a track segment and any loss of power or disconnected wire results in a safe signal indication to the train. Track circuits also allow for the use of clips that instantly shunt the circuit and mark the track as occupied. These can be used by crews or maintenance personnel to quickly report an unsafe condition or mark a section of track out of service. Modern axle counter equipment transmits data from the trackside apparatus to the indoor equipment via telegrams, across an ISDN line. This results in the section of line being monitored showing occupied in the event of persisting technical fault or loss of telegrams. The section then requires a reset command and further interaction to restore to service.

Broken rails

The track circuit provides additional functionality of detecting many, however not all, kinds of broken rails, though only to a limited extent in AC traction areas and not in the common rail in DC traction areas. Axle counters offer no such facility. However, experience has shown that broken rails often occur near the insulated block joints which are used to electrically isolate adjacent track circuits. Since axle counters do not require such block joints, the risk of having a broken rail is significantly reduced.

Siding and shunting movements

Axles counters have problems maintaining correct counts when train wheels stop directly on the counter mechanism. This can prove problematic at stations or other areas where cars are shunted, joined and divided. Also, where main lines have hand operated switches to siding, spur or loop tracks the use of counters is more costly to implement to detect trains appearing and disappearing from a track segment.

Electromagnetic brakes

Magnetic brakes are used on high speed trains (maximum speed greater than 160 kmh). These are physically large pieces of metal mounted on the bogie of the vehicle, only a few centimetres above the track. They can sometimes be mistakenly detected by axle counters as another axle. This can happen only on one side of a track block, because of magnetic field curvature, defects of track geometry, etc., leading the signalling system into confusion and also requiring reset of the detection memory. The modern AzLM variant of axle counter is 'eddy current' brake proof and the magnetic effect of the braking system described above is overcome, therefore count information remains stable even when a vehicle fitted with magnetic brakes is braking whilst traversing the rail contacts of a detection point.

Reset and restoration

There are four methods of securing the reset and restoration of axle counters into service:

  • Preparatory reset uses the internal logic of the axle counter system to enforce that a train must proceed through a reset section at slow speed, by holding its output as 'occupied' until the train is successfully detected as passing through the section. This logically proves the section free of obstruction and therefore allows the section to change its output to 'clear'. It is problematic for a long track section due to the long time required for the train to pass at a slow speed.
  • Conditional reset (with aspect restriction) has the section reset only if the last count was in the outward direction. This at least shows that any trains in the section at time of reset were moving out. The signal protecting the reset section is held at danger by signalling logic outside of the axle counter evaluator to enforce a low speed 'sweep' of the section prior to restoration to service.
  • Un-conditional reset (no aspect restriction) has the section reset irrespective of the the last count action. The protecting signals are cleared immediately after a reset. In the UK, this type of reset is used under 'EPR' 'Engineer's Possession Reminder' and a series of procedures are carried out to ensure the section of line is clear of vehicles and tools before the reset is performed.
  • Co-operative reset requires both the technician and signaller to co-operate to reset and then restore the section into service. This type of reset is now only used on schemes which fringe on an existing scheme which utilizes this type of reset arrangement.

Most countries use a variation of the above four methods, sometimes with varying amounts of automation or human input.

See also

Makes

References


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