In general, a Rail WIM system will measure the deflection of the rail – or a part of the rail bridge – during the passage of a train. From the sensor signals the dynamic and static axle loads, bogie load (axle group) and GVW of each of the cars of the train are determined. Different sensing techniques are being used, like strain gauges, piezo-quartz or fibre optic devices. In most cases the sensors are attached directly to the rail, except for Bridge-WIM, where they are attached to the structure itself.
Compared to the road WIM systems, the rail variations generate results with higher accuracy and reliability, with typical inaccuracies of ±2 % for GVW and ±5 % for axle load measurements. This is caused by two things:
- As the wheels of the train strictly follow the rails there is no variation in the lateral position of the train when it passes over the sensors. This reduces the disturbances caused by lateral movements (by steering) and makes the measurements more repeatable.
- Locomotives, especially the electric ones, have a fixed and known weight. This can be used for automatic calibration, monitoring and improvement of the measurement accuracy. The locomotives can be detected based on the combination of axle configuration and weights or through electronic or optical identification of individual locomotives.