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An axle counter is a railway system safety device that detects when trains pass, or more importantly, remain on, a specific track section. This detection functionality is achieved by sensing or counting each set of wheels or “axle” which passes over a counter pair. Axle counters are installed as part of the signaling systems on railways and serve to establish whether or not critical track sections are occupied or free of traffic. This information may be relayed to signal control points which in turn switch the system signals accordingly. This system has several advantages over conventional track circuit installations including a lack of insulated track joints and minimal cabling.
The on-track equipment for an axle counter installation consist of two detection points placed at either end of a track section. Each detection point is fitted with two independent sensors, thereby allowing the detector to establish both the passage of and direction of travel of rolling stock passing over it. Each time a detection point senses the passage of a set of train wheels, or axle, the circuit increments a counter by one. The total axle count accumulated by the first detector is temporarily stored until the train reaches the second detection point. Each time the second detector is activated, it decrements the total by one; if the net result is zero, the track section is deemed to be clear.
These calculations are carried out by computerized control units located at central points or in track-side kiosks. The signals from the individual detector sets are transmitted to computers by conventional copper cables or via telecommunications links. Axle counter systems require no peripheral equipment at the detection point, thus making them ideal for installations in isolated, difficult-to-maintain locations. Another advantage of these systems is that no insulated joints between individual track sections are necessary as is the case in track circuit detectors. The axle counter setup is also generally much cheaper due to the lack of additional track-side equipment and the reduced amount of cabling involved.
One of the few disadvantages of axle counter systems is their tendency to lose stored data in the event of a power failure. This generally requires a manual override function with associated human reliability issues. Current developments which incorporate ISDN transmitted telegrams into the existing system architecture show great promise in overcoming this type of failure. Other disadvantages include the system's inability to detect broken rails and the return of erroneous counts on high speed trains with bulky magnetic brake units mounted on the axle assemblies.