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Guest article

Absolute track measurement

One of the tasks of the permanent way staff of the Italian railway network (RFI, Rete Ferroviaria Italiana) is checking the absolute track geometry. This ensures that the geometric quality of the track meets the increasing speed. RFI has introduced new control techniques over the last few years to make this work easier and faster.

Guest article
Dipl.-Ing. Paola Conti, Department Head for Maintenance, Infrastructure and Civil Engineering

Traditionally, rail sections fixed in the ballast at the side were used for fixed-point marking, with the distance of the rail sections to the track and to each other being specified in the RFI regulations. These reference markers can shift over time, for example due to maintenance work such as ballast cleaning.

When high-speed trains, the AV/AC (Alta Velocità/Alta Capacità meaning high speed and high capacity), were introduced in Italy, RFI decided in the construction phase to position the rails in relation to georeferenced fixed points. The absolute basis system (AB) consists of fixed points (bolts) which are fitted to overhead line masts and to tunnel walls. Their distances to the centre of the track on the line are known and defined by topographical coordinates (x, y, z). This system makes it possible to monitor the track geometry in relation to its original position, i.e. how the track has changed or developed since construction, and it also supports maintenance work being carried out with tamping machines. The benefits of this approach: deterioration of the track geometry is greatly reduced, ride comfort is noticeably improved, costs and frequency of maintenance work are reduced.

The experience gained over the last few years from track geometry management using absolute track measurement on the AV/AC lines encouraged RFI to extend this control method to all electrified lines. Thus, a gradual phasing out of the traditional system was stipulated from 2014 and provides AB equipment on lines as part of track renewal work. This will cover about 12,000 km of track by 2025.

For a fast distribution of the AB system it is essential for RFI to procure systems with which the measurement of the absolute track geometry can be carried out ever faster and more easily.

Currently, manual topographic measuring trolleys, self-propelled measuring vehicles (track recording car EM-SAT 100) or intelligent tamping machines are used as control tools. With these the qualified staff of RFI is able to measure with high accuracy the position of the track relative to the fixed points by comparing the construction data with the actual position of the track, with productivity limited to two to three km/h.

To overcome this limitation, RFI has started to cooperate with Plasser & Theurer during the last few years to investigate a new method for the absolute measurement of the track geometry. The aim is to carry out measurements at a speed of at least 60 km/h using the inertial measurement system installed in some tamping machines.

Using this “latest-generation tool”, the geometric profile of the track can be determined at any speed – the tamping machine becomes a diagnostic tool for the relative track geometry. To make this measurement absolute, a system was developed, on-board a first tamping machine in Bologna, which can detect the spacing of the track at fixed points during operation of the machine. Using stereo cameras which are part of the equipment, it is possible to measure with high accuracy markers installed permanently at fixed points. This solution simplifies the absolute measurement of the track and achieves a noticeable increase in productivity which is essential with regard to the expansion of the rail network on an absolute basis.