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 Preparation of the Track

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The Test Tracks: an Overview

The test runs took place on a section of the Atlantique branch of the TGV network, a few months before the line was opened to TGV revenue service. Strictly speaking, there were no significant alterations of the track or catenary for testing purposes. However, some sections of the line's profile had been planned since 1982 (shortly after the TGV Sud-Est world speed record of February 1981) to allow very high speed running.

Construction of the dedicated tracks of the TGV Atlantique was officially decided on 25 May 1984. Ground was broken on 15 February 1985. The new line was to stretch from slightly outside the Gare Montparnasse in Paris to Le Mans, with a second branch towards Tours. The Le Mans branch was opened for 300 km/h (186 mph) revenue service on 20 September 1989, and the Tours branch opened a year later. The two branches separate at Courtalain, 130 km west of Paris, where movable frog switches good for 220 km/h (137 mph) in the diverging route direct trains towards either Le Mans or Tours.

The test section itself begins on the common branch, at kilometer 114, at the Dangeau siding. It runs past Courtalain and onto the Tours branch of the line. Between kilometer 135 and kilometer 170, the line was designed with progressively wider curves, reaching a minimum radius of 15 km (9 mi) after kilometer 150. These curves were built with larger superelevation than strictly necessary for revenue running at 300 km/h (186 mph). At kilometer 160, the line passes through the Vendôme TGV station. At kilometer 166, there is a long 2.5% downhill stretch into the Loir valley (the Loir is a tributary of the better-known Loire river) and the line crosses the Loir on a 175 m (1100 ft) bridge. This is the area where the highest speeds were expected, and most of the activity was concentrated there.

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The Tours branch of the line was tested by special computerized Maintenance of Way equipment, from the Track Research department of SNCF. Just as on all TGV lines, the rails were aligned to 1 mm (3/64 in) tolerances, and the ballast was blasted to remove small, loose gravel. In subsequent testing with trainsets 308 and 325, the track was not significantly affected and required only minimal realignment. This was in contrast to the 1955 world speed record of 331 km/h (206 mph), also set in France, where the track was seriously damaged after the high speed runs. strain gauges were placed in several locations, especially at the expansion joint at the end of the Loir bridge.

The catenary was standard TGV style, without any modifications. The only changes were in the tuning. TGV catenary is strung in 1200 m (4000 ft) sections, mechanically tensioned by a system of pulleys and couterweights. Support masts are spaced at 54 m (175 ft) intervals. The catenary (supporting) wire is made of bronze, with a circular cross section of 65 square millimeters. The contact wire is made of copper, and has a cross section of 150 square millimeters. The cross section of the contact wire is circular with a flat section on the contact side.

In general, when a pantograph runs underneath the catenary, it sets up a wave-like disturbance which travels down the wire with a speed determined by the tension in the wire and its mass per unit length. When a train approaches this critical speed, the pantograph catches up with the disturbance, resulting in dangerously large vertical displacements of the wire as well as contact interruptions. The top speed of the train is then limited by the critical speed of the catenary. This problem was very central to the test runs, since it was desired to test 325 at speeds well above the critical speed of standard TGV catenary. There were two solutions: increase the tension in the wire or reduce its mass per unit length.

Replacing the copper contact wire by a lighter cadmium alloy wire was considered, but dismissed on the grounds of time and cost. The critical speed of the test track catenary was then to be increased solely by increasing the tension in the wire. For the test runs, the usual tension of 2000 daN (4500 lbf) was increased to 2800 daN (6300 lbf) and exceptionally 3200 daN (7200 lbf). For some of the faster runs over 500 km/h (311 mph), the voltage in the catenary was increased from the usual 25 kV 50 Hz to 29.5 kV.

At kilometer 166, catenary masts were equipped with sensors to measure the displacement of the wire. During the 18 May 1990 record at 515.3 km/h (320.3 mph), vertical displacements of almost 30 cm (1 ft) were recorded, and fell within 1 or 2 cm of the predictions made by computer simulations. The critical speed of the catenary for that particular run was 532 km/h (331 mph).

Pictures from La Vie du Rail

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Picture 1: The section of track where the records were set. Catenary masts in the foreground are equipped to measure contact wire displacement.
Picture 2: Technicians installing strain gauges on the expansion joint located at the end of Loir bridge.

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Picture 3: Inspection and tensioning of the catenary. The counterweights are hung along the catenary mast, and tension the wire through the system of pulleys.
Picture 4: Spot realignment of the track after one of the runs.

Photo Credits: Picture 1, P. Olivain/CAV-SNCF; Picture 2, M Barberon/LVDR; Picture 3, M. Barberon/LVDR; Picture 4, M. Barberon/LVDR