A high-speed line (HSL) is designed specifically to be suitable for TGVs running at 300 km/h (186 mph). Its construction is basically similar to that of conventional lines, but the specifications, and in particular the precision and allowable tolerances on safety-critical elements, require much greater care and attention to be applied at all stages of the work. 
In most cases, the high-speed lines run across open country between suitable points on the conventional rail network situated on the outskirts of major cities, co-located wherever possible with existing infrastructure such as motorways. In Belgium, the TUC-Rail company is the Master of Works for the implementation of the High-speed line programme.

Preliminary studies

Once the route of a proposed high-speed line has been chosen, and the necessary authorisations obtained, the first task is to measure the geotechnical characteristics of the terrain along the corridor, as this will indicate what special construction techniques, if any, are needed. If the subsoil is sandy, for example, it will probably be necessary to stabilise the track platform by means of a layer of tarmacadam or concrete.

Archeological research

Archeological research along the HSLThe next stage is the search for and excavation of sites of archeological interest. In Belgium, the cost of this, averaging a million BEF per km, is funded by the SNCB. Sites of interest are mainly located by inspection along the route, but some historical relics have been found by the construction crews, who are also on the look-out for artefacts. The oldest relics have been dated at more than 2000 BC !

Civil engineering

Once these preliminaries have been dealt with, the civil engineering work such as earth-moving and bridge-building can begin.

Construction of a viaduc at LembeekIdeally, the work due to take longest is started first, so that everything will be ready simultaneously, but often priority must be given to providing access to the working sites. As far as possible, the earth removed in excavating cuttings is used as fill for embankments. Where the load-bearing strength or stability of the subsoil is insufficient, it must be reinforced by adding hardcore, or even pile-driving. All but the smallest bridges are now built with reinforced concrete, though some are clad in stone or brick for environmental reasons. Where possible, bridge costs are minimised by locating minor road-crossings where advantage can be taken of a natural difference between road and rail level, even if this involves a short detour for road traffic.

In all cases, the final result is a firm platform forming a corridor 14 metres wide, ready for the next stage: installation of the railway infrastructure.

Railway infrastructure

Finally, then, the railway infrastructure itself takes shape, in a sequence of three stages: track-laying, electrification and signalling.


Track-layingFirst of all, prefabricated sections of temporary track each 18 metres long on wooden sleepers are assembled into a continuous whole along the site of one of the future lines. These tracks are used initially by trains bringing loads of crushed stone which form the basic layer of ballast when spread evenly across the track-bed.

Then, diesel-hauled track-laying trains bring in the material for the adjacent permanent track : pre-tensioned concrete sleepers each 2.4 m long and weighing 245 kg and rails in 288 metre lengths. Once the sleepers have been aligned on the ballast a travelling gantry is used to install the rails onto them, at the correct spacing. Finally, the lengths of rail are welded together in situ to eliminate the need for mechanical joints.

The first trains to run over this permanent track bring more ballast, which is spread evenly up to the level of the top of the sleepers. Then rail-borne tamping machines are used to ensure that each sleeper is firmly embedded at exactly the right position in the mass of ballast, so that it will remain in place even after many high speed trains have passed.

Ballast layingMeanwhile, a similar sequence of events on the other line, beginning with the removal of the temporary track for re-use elsewhere, results in the completion of the double-track high-speed route. As each track is effectively independent of the other and without signalling at this stage, all construction train movements must be planned carefully in advance.


Electrification of the lineOnce the permanent tracks have been laid, the equipment needed for electrification is brought to the working sites and installed, in a series of steps.

This begins with installation of the masts to support the catenary, which in most cases are inserted by a travelling crane into holes drilled at the edge of the track bed, and held in position by stays until the concrete poured into the holes afterwards has set. Next, the various insulators, brackets and fittings needed to keep the catenary and contact wire located correctly above the tracks are assembled and attached to the masts. Finally, the wires themselves are installed and adjusted under the tension of 14 kN applied longitudinally so that the vertical displacement of the contact wire does not exceed 12 cm.

After all adjustments have been made, the power can be switched on. It is transformed down to the standard 25 kV 50 Hz at a sub-station located at a point where the public grid is conveniently accessible. Sub-stations are located about 40 km apart, so that normal traffic can continue even when one is out of operation for any reason. This distance is much greater than is possible with the 3000 V DC of the national network in Belgium.

Signalling for the high-speed lines

Signalling systemInstallation of the signalling equipment is the last stage of the job. Because the drivers of high-speed trains wouldn't have the time to keep track of conventional lineside colour-light signals, and they need information that those signals can't display, the high-speed lines are equipped with a more advanced cab-signalling system.
Two such systems are in use: that known as TVM on line 1 and TBM on the other lines.

The signalling on all high-speed lines in Belgium is controlled from a central signal box (Block 12), located in the SNCB offices in rue Bara, Brussels.

For further information on signalling technology, see the page about the signalling of high-speed lines.


Maintenance base of line 1As well as needing special care during constructing the high-speed line, the track geometry must be verified regularly once it is in operation. Routine maintenance is therefore undertaken, involving both visual inspection on the ground and making measurements in special vehicles while on the move. 

Whereas there are usually plenty of points on conventional railways where a train with a defective coach or wagon can be diverted off the running lines, refuge sidings are provided specially for this purpose on the high-speed lines (usually on level ground beside a pair of crossover tracks and the associated signalling relay cabin). These refuge sidings are also used to store maintenance vehicles while away from their base, which was typically a construction site originally (Le Coucou on line 1, near Ath, and Voroux on line 2, near Ans) and may accommodate equipment for use in emergencies.

Furthermore, every fortnight the track geometry, the state of the catenary and communications facilities are measured by means of the SNCF's TGV-compatible inspection coach, known as "Melusine", which is inserted between the power car and the first coach of a TGV-R. Despite its TGV livery, Melusine can be identified easily because it has a unique observation dome.

Back to the home pageReturn to the index to the TGV section.

This page is maintained by David De Neef. Translation by Alan Reekie. It has been seen by visitors since 12 August 2000.
Last revised on 02/02/02 .

Main sources of information: