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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.
||The factual accuracy of part of this article is disputed. The dispute is about when axle counters were first introduced.. (October 2011)|
Axle counters were first introduced in the 1960s in Germany by Siemens. 
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.
Axle counters are particularly useful on electrified railways as they eliminate traction bonding and impedance bonds. 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 (such as the Forth Bridge), which may prevent the normal operation of track circuits if insulating the rails from the structure proves impracticable. Axle counters are also useful on long sections where several intermediate track circuits may be saved. A Frauscher axle counter sensor, for example, can be 8,500 m from the evaluation unit, while the latest ALTPRO axle counter sensor model ZK24 can even go up to 49 km from the unit.
The axle counter cable of 8,000m or 49,000m would typically be buried in a plastic conduit, which can also be used for CBI cables. The conduit would have termination boxes every few thousand feet to assist in fault finding.
In the case of Frauscher axle counters, the cables have four cores: two for power (positive and negative), and one each for counting in each direction. In case of ALTPRO ZK24 axle counters, where ALTPRO VUR module is used, the cable requires only two cores: power (positive and negative) while the signals from the axle counter (from the two sensor's heads) are sent back modulated over the very same core used for the power supply.
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. In older installations the evaluators may use eight-bit logic, causing numerical overflow when a train with 256 axles passes the axle counter. As a result, this train will not be detected. This imposes a length limit of 255 axles on each train.
Where there are interlocked turnouts, an axle counter unit needs to be provided for each leg of that turnout. On lines with non-interlocked/hand operated switches, detection of the switch points would have to be monitored separately, whereas on track circuited lines misaligned points can be set to automatically break the track circuit.
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 restrictive 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. Some manufacturers provide axle counter equipment which is fail safe in design. New technology allows for occupancy detection if the axle counter detaches or becomes loose from the rail, has a conductor open or short condition, and with some designs that use dual internal sensors within the axle counter will show occupancy when only one system is working within the axle counter by activating based on number of pulses detected from the remaining good system inside the axle counter.
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. Ordinary track circuits have a blind spot of about a metre in length from the wiring connections to the insulated joint.
Siding and shunting movements
Axle counters have problems maintaining correct counts when train wheels stop directly on the counter mechanism; this is known as 'wheel rock'. This can prove problematic at stations or other areas where cars are shunted, joined and divided. Also, where main lines have switches to siding, spur or loop tracks extra counters will need to be deployed to detect trains entering or exiting the line, where with track circuits such infrastructure needs no special attention.
In Auckland, New Zealand, axle counters have been used on all lines where track circuits are required except for special places where Hi Rail maintenance vehicles either on or off track. All road crossing tracks at public level crossings are deemed to be Hi Rail access points and a short single rail DC track circuit is used. There are also several single rail DC track circuits at places not at level crossings where Hi Rail vehicles can access the track.
Magnetic brakes are used on high speed trains (maximum speed greater than 160 km/h). 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, or other issues, 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 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.