Axle counter
An axle counter is a system used in railway signalling to detect the clear or occupied status of a specified section of track.
The axle counter sensors are either connected to the evaluator via dedicated copper cable or via a telecommunications transmission system.
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.
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 due to rust, grease or compacted leaf residue.
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.
An accident which occurred in the Severn Tunnel is thought to have been due to the improper restoration of an axle counter.
That imposes a restriction against multiples of 256 on axle count - with operators typically adding additional empty wagons to avoid the exact number.
Ordinary track circuits do have a blind spot of about a metre in length from the wiring connections to the insulated rail joint (IRJ).
That is known as 'wheel rock', and can prove problematic at stations or other areas where cars are shunted, joined and divided.
That is quicker and easier to mount in the right conditions, but can mean more frequent checks to ensure correct positioning is maintained.
However, they were susceptible to errors and were replaced in Europe at the end of the 19th century by hydraulic rail contacts.
[7] In pneumatic axle counting systems, pistons were actuated by specific loads and speeds.
The first US patent for an axle counter, filed on 3 June 1960 by Ernst Hofstetter and Kurt Haas,[8] was for a device of this type.
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.
