An axle counter is a device on a railway that detects the passing of a train between two points on a track. A counting head (or 'detection point') is installed at each end of the section, and as each train axle passes the counting head at the start of the section, a counter increments. A detection point comprises two independent sensors, therefore the device can detect the direction and speed of a train by the order and time in which the sensors are passed. As the train passes a similar counting head at the end of the section, the counter compares count at the end of the section with that recorded at the beginning. If the two counts are the same, 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.
- 1 Applications
- 2 Advantages
- 3 Disadvantages
- 4 Installation
- 5 Reset and restoration
- 6 History
- 7 See also
- 8 References
- 9 External links
Track vacancy detection
The most common use for axle counters is within railway signalling for track vacancy detection. This is a form of block signalling, which does not permit two trains to be within the same block at the same time, this decreases the chance of collision due to railways being divided into blocks which ensures there is always enough space between trains to allow one to stop before it hits the one in front.
Axle counters are also used for the switching on and switching off of railway crossings. Closing the crossing to pedestrian and motor vehicles when the presence of a train is detected, and allowing them to open back up when the successful traverse of the train has been recorded.
Switch protection in rail yards
Axle counters are used in rail yards to detect train cars as they are sorted. Axle counters are placed on the track before each switch and on each track that exits the switch. Rail yard management software uses occupancy data from the axle counters to lock in switches and prevent cars from being routed to tracks that are occupied by other cars.
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 are also useful in instances were there are issues with water on the track. Generally wheel sensors work under water, as long as the wiring is above the water.
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. This is because the operation of axle counters does not rely on the contact of wheel with the rail head to provide an electrical circuit.
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 Thales axle counter sensor, for example, can be 10,500 metres (34,400 ft) from the evaluation unit when connected using the ISDN communication only, but with the addition of an Ethernet converter the distance is limited by the transmission system while the latest ALTPRO axle counter sensor model ZK24 can even go up to 49 kilometres (30 mi) from the unit.
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. Modern systems are not restricted by train wheel numbers.
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.
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 \ higher speed trains (maximum speed greater than 160 km/h (99.4 mph)). 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 axle counters are '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.
The two main methods of mounting an axle counter is firstly drilling through the rail, this is seen as time consuming and possible damaging to the rail. However this eliminates the need for leveling which can help reduce maintenance costs.
The second is a rail mount, which clamps to both sides of the rail from underneath it. This can be seen as quicker and easier to mount in the right conditions, but can mean more often checks needs to be made to ensure the correct positioning.
Reset and restoration
There are four methods of securing the reset and restoration of axle counters into service:
- Preparatory reset once a Preparatory reset is applied to the system, the axle counter continues to show the section as occupied until one train movement takes place in the section, logically if a train has successfully traversed the section, then the section is clear and the axle counter is set back to unoccupied.
- Conditional reset 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 occupied until a sweep or physical verification of clearance of the track.
- Un-conditional reset 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.
The first US patent for an axle counter was filed on 3 June 1960 by Ernst Hofstetter and Kurt Haas.
Axle counting initially started with treadle-like mechanisms. They consisted of a mechanical contact device mounted on the inside of the foot of rail; the wheel flange running over the device actuated a lever. However they were susceptible to errors and were replaced in Europe at the end of the 19th Century by hydraulic rail contacts.
Hydraulic rail contacts were actuated by the deflection of the rail caused by axle load running over the tracks. The first cylinders were filled with mercury; later hydraulic oil was used. They were then replaced by pneumatically operated switching elements.
In pneumatic axle counting systems, pistons were actuated by specific loads and speeds. They proved limited in application, and therefore from the 1950s onwards were replaced by magnetic contacts. Up until that point, Track Circuits always had a big edge when it came to reliability.
Magnetic contacts were the first contactless switching devices. They were known as "axle counting magnets". The iron wheel flanges triggered an actuation by interrupting a magnetic field. Hofstetter and Haas' patent was of this type. During this time, inductive methods were also being produced based on transformers. During the 1970s, developments in the electronics field as well as the introduction of integrated circuits allowed the design of the axle counters currently used.
- List of rail accidents
- Severn Tunnel rail accident (1991)
- Rail inspection
- Railway signalling
- Track circuits
- "THE DEVELOPMENT AND PRINCIPLES OF UK SIGNALLING". Railway Technical. Retrieved October 21, 2014.
- "BO23 Brochure" (PDF). AltPro. AltPro. Retrieved October 21, 2014.
- "Introduction to Tiefenbach Wheel Sensor Technology". Tiefenbach. Retrieved October 21, 2014.
- "Drill Free Axle Counter Mounting Bracket". Argenia. Retrieved December 15, 2014.
- "DIGITAL AXLE COUNTER" (PDF). CAMTECH. April 2010. Retrieved October 21, 2014.
- "Axle counter for railroad installations, US 3015725 A". Google patents.
- Rosenberger, Martin (2012). "Future Challenges to Axle Counting Systems" (PDF). IRSE. Retrieved October 21, 2014.