interesting info from wikipedia.............
DIGITAL AXLE COUNTER
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...
more... 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.
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 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 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
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 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 by a separate system. On track circuited lines misaligned points will automatically break the circuit path.
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 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.
