An axle counter is a system used in railway signalling to detect the clear or occupied status of a section of track between two points. The system generally consists of a wheel sensor (one for each end of the section) and an evaluation unit for counting the axles of the train both into and out of the section. They are often used to replace a track circuit.
Principles and operation
An axle counter consists of a sensor, which detects the individual axles of a train by mechanical, electrical or even fibre optic method, and an evaluator which counts the axles passing into and out of a rail section. The evaluator may also convert the analogue signal of the axle sensor into a digital signal. However, in some cases there is a separate unit which performs this task.
The system is set up by having an axle counter sensor installed at each end of a section. As each train axle passes the axle counter sensor at the start of the section, a counter increments. An axle counter sensor comprises two independent sensors (so 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 axle counter sensor at the end of the section, the system 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.
This process is carried out by safety-critical centrally located computers, called "evaluators", with the axle counter sensors located at the required sites in the field. The axle counter sensors are either connected to the evaluator via dedicated copper cable or via a telecommunications transmission system. That allows the axle counter sensors to be located a significant distance from the evaluator, and is useful when using centralised interlocking equipment, but less so when signalling equipment is situated beside the line in equipment cabinets.
Applications
Track vacancy detection
Railway signalling
The most common use for axle counters is in railway signalling for track vacancy detection. It is a form of block signalling, which does not permit two trains to be within the same section of track (block) at the same time. Block signalling decreases the chance of collision, because dividing the track into blocks ensures there is always enough space between trains to allow one to stop before it hits one in front.
Railway crossings
Axle counters are also used to switch on and switch off warning equipment at level crossings, closing the crossing to pedestrian and motor vehicles when the presence of a train is detected, and allowing them to open when the train has passed over the crossing.
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 switches and prevent cars from being routed to tracks that are occupied by other cars.
Advantages
Unlike traditional track circuits, axle counters do not require insulated rail joints to be installed. This avoids breaking the continuity of long sections of welded rail to allow insulated joints to be inserted.
Electrified Railways
Axle counters are particularly useful on electrified railways as they eliminate the need for 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.
Insulated Rail Joints
Axle counters eliminate most railjoints (IRJ) which are a weak point on the track.
Railhead Contamination
Axle counters do not suffer problems with railhead contamination due to rust, grease or compacted leaf residue. Sometimes a train puts down anti-slip sand to aid deceleration while braking, but the sand contaminates the rail and the track circuit stops working. Axle counters are immune to those problems because they do not rely on the contact of wheel with the rail head to provide an electrical circuit.