Axle counter
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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 a head at the start of the section, a counter increments. As the train passes a similar counting head at the end of the section, that 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. These 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.
[edit] Advantages
Axle counters are used in places such as wet tunnels, like the Severn Tunnel, where ordinary track circuits are unreliable. Axle counters are also useful where there are uninsulated steel sleepers which prevent the 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.[citation needed] 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.
[edit] 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 too. An accident occurred in the Severn Tunnel due to improper resetting of the axle counters.
There are three methods of securing the reset and restoration of axle counters into service:
- Co-operative reset requires both the technician and signaller to co-operate to reset and then restore the section into service. This directly manages the cause of the Severn Tunnel accident.
- 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'.
- 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 at least 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.
Most countries use a variation of the above three methods, sometimes with varying amounts of automation or human input.
[edit] Broken rails
The track circuit provides additional functionality of detecting many, though 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, however, 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 rails is significantly reduced.
