An axle counter is a device on a railway that detects the passing of a train

Train

A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

 in lieu of the more common track circuit

Track circuit

A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...

. A counting head (or 'detection point') is installed at each end of the section, and as each axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the 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

Transmission system

In telecommunications a transmission system is a system that transmits a signal from one place to another. The signal can be an electrical, optical or radio signal....

. This allows the detection points to be located significant distances from the evaluator. This is useful when using centralised interlocking

Interlocking

In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...

 equipment but less so when signalling equipment is distributed at the lineside in equipment cabinets.

History

Axle counters were first introduced in the 1960s in Germany.

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.

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 tunnel

Tunnel

A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...

s (such as the Severn Tunnel

Severn Tunnel

The Severn Tunnel is a railway tunnel in the United Kingdom, linking South Gloucestershire in the west of England to Monmouthshire in south Wales under the estuary of the River Severn....

), where ordinary track circuits are unreliable. Axle counters are also useful on steel structures (such as the Forth Bridge

Forth Bridge

The Forth Bridge is a cantilever railway bridge over the Firth of Forth in the east of Scotland, to the east of the Forth Road Bridge, and 14 kilometres west of central Edinburgh. It was opened on 4 March 1890, and spans a total length of...

), which prevent the normal operation of track circuits. 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 8,500m from the evaluation unit, while the latest ALTPRO axle counter sensor model ZK24 can even go up to 49 km from the unit.

Cables

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

Computer-based interlocking

Computer Based Interlocking is the general term for a railway signalling interlocking implemented with computers rather than the older technologies such as relays or mechanics.- General :...

 cables.

The cables have four cores: two for power (pos and neg), and one each for counting in each direction.

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.

Turnouts

Where there are turnouts, an axle counter unit needs to be provided for each leg of that turnout.

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. 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

Axles 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 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.

In Auckland, New Zealand, axle counters will be used on the main lines, but any headshunts connected to sidings will use ordinary track circuits.

Electromagnetic brakes

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
  Signalman (rail)
  A signalman or signaller is an employee of a railway transport network who operates the points and signals from a signal box in order to control the movement of trains.- History :...
  
   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.

Makes

Argenia Railway Technologies Inc.  Celindia  Frauscher  - ACS2000 Siemens

Siemens

Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...

  Thales Group

Thales Group

The Thales Group is a French electronics company delivering information systems and services for the aerospace, defense, transportation and security markets...

  GE Transportation  Stesalit Ltd.  GGTronics.  CEL  Duvine  Zvonimir Viduka - Croatia ALTPRO - Croatia Scribd

Scribd

Scribd is a Web 2.0 based document-sharing website which allows users to post documents of various formats, and embed them into a web page using its iPaper format. Scribd was founded by Trip Adler, Tikhon Bernstam, and Jared Friedman in 2006...

 

External links

• Electropedia
• ARTC
  Australian Rail Track Corporation
  Australian Rail Track Corporation is a federal government owned corporation established in 1997 that owns, leases, maintains and controls the majority of main line standard gauge railway lines on the mainland of Australia, known as the Defined Interstate Rail Network .-History:The Interstate...
  
  level crossing example .