Track Displacement and Tilt
The key concern when assessing a railway line is whether it is fit for purpose – will it carry a train at the designated line speed safely? Of secondary importance are a number of other factors including ride comfort and assessing future maintenance requirements. Prior to Imetrum's measurement system, there has not been a simple way of accurately assessing the movement of the track as a train passes over.
Our measurement system has been successfully used in a number of European Countries to gather this information for points, individual joints, 3 m sections and longer lengths of track. The software has used images from our camera to track the movement of the joint bolts, sleepers and chairs – all this can be done without accessing the track. Simple painted or vinyl self-adhesive targets have also be fitted to the rail to determine movement at specific locations, both for expansion near joints and vertical deflection under train load investigations. As a dynamic tool, it is quick to set up, can be operated from a safe distance from track, and is accurate to better than 0.1 mm.
Imetrum's system can be used both for monitoring during and after nearby construction work, for a general track assessment or to more accurately determines tilt, twist and deflection where a problem has been identified. It can be tripod-mounted for quick assessments, or bolted to a solid plinth or piece of trackside furniture for a long term application. The system can either output data continuously, or else be triggered at intervals (hourly, as a train approaches etc). For monitoring lengths of track, video of each area of interest can be recorded on site, then post-processed back in the office to obtain the required measurement data.
Our system does not need targets to work; indeed it can often be used from outside of the permanent way altogether. Measurements can therefore often be taken without the need for a possession, or employing a COSS or Lookouts, leading to substantial savings over existing techniques. Its sub 0.1mm accuracy, and multi-point dynamic measurement can lead to the production of richer data than has previously been available.
As trains pass over any given location, they cause a movement of the rail surface. This is primarily down to flexure of the rail, compression of the ballast, and voiding under the sleepers. Of primary concern to the railway engineer is what the overall deflection of the rail is, in order to assess the potential tilt of the track and hence derailment risk. The Imetrum system can directly measure tilt of a 3 m track section, without a need to get trackside. It can also be used as a simple displacement measure.
The traditional method of determining track deflection is a void meter. This is a simple, robust device that depresses a ring down a metal shaft, therefore recording the maximum displacement. These are easy to position, but do need access to track – something which the Imetrum system avoids. The void meter rests on the ballast, and so is not able to take account of any movement in the ballast itself as the train passes.
The results of our investigation show that ballast movement is significant. In this case, around 25% of total movement is due to compression of the ballast. This means that the void meter will be under-reading, potentially indicating that a track deflection is within an allowable limit, when in fact it is not. Additionally, the void meter was only able to pick up a maximum deflection, whereas the Imetrum technology accurately measured the dynamic displacement, picking up 1mm of heave as well as the deflection of the track under each axle load.
Safety Critical Data
Taking into account ballast deflection and heave, overall track movement was actually 5 mm peak-peak, as opposed to the 3mm indicated by the void meter. The implications of a measurement inaccuracy on this scale could be serious. Lines passed as being in good condition could in fact carry a risk of derailment. Using the Video Gauge, engineers and surveyors are able to generate more accurate data than previously, without the risks and time penalties associated with accessing live rails.