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24.07.15

Detecting track defects

Source: P.C Johnson

In the 1970/80’s various test trains were developed as individual facilities to assess various track features and brought into service as separate operations. Four of these major units were the Ultrasonic Test Unit, Track Recording Unit, the Structure Gauging Train and Lab 5, a track Recording Coach already in long term use. Even at that time there were thoughts of eventually combining these operations into a possible single unit to reduce line occupancy, costs and some of the duplicated staffing aspects. In a combined arrangement the speed of testing would be determined by the test requiring the lowest testing operational speed requirement and possible light level requirements.

The track recording facilities could work at any speed up to line speed with a constraint of a comparatively slow minimum low speed. The installation of track recording facilities as a unit on conventional ‘in traffic’ stock gives some aspects of the track being travelled over but does not give specific detailed information and tends to be influenced by the riding characteristics of the vehicle conditions on which it is installed. The Structure Gauging facility worked at similar but preferably slightly lower speeds principally at night. The Ultrasonic facility had the lowest speed requirement determined by the time taken for the transmitted shear wave ultrasonic energy to return from the bottom of the rail and the pulse spacing to ensure full coverage. This restricts the speed of operation to around 20 to 30mph.

My recollection of detected in track rail defects and failures indicated in general they were usually located in the upper half of the rail. Rail-end defects in the lower web were usually associated with other defects in the upper half of the rail. The few other true rail foot defects usually occurred in known galling and corrosion areas and could be accommodated accordingly.

Testing down to mid-rail depth would allow a significant increase in testing speed. However this approach would not be without difficulties, needing to ensure residual energy from consecutive events did not interfere. With an increase of speed it would be necessary to maintain the density of pulsing to ensure full examination coverage. However with the current computing facilities available and a designed multiple probe arrangement it should be possible to produce a system to overcome these problems.

Work to enhance the fracture toughness of rail steels would also improve defect resistance to failure. A more extensive use of the more expensive, but probably cost effective, rolled High Manganese steel rails, in vulnerable areas, with their tolerance of significant cracks without failure. Although this material does carry penalty of a lower propagation speed of ultrasonic energy adding to difficulties in automated processes.

Filming of track and pantograph features could also be incorporated with its own constraints. In consequence I would suggest it should be possible, with careful planning, to carry out several measurement features on the same train/unit simultaneously reducing costs and track occupancy.

Re: Current IM fleet data collection service 'unsustainable'

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