Brussels) or where isolated outlying lines have been preserved (as on the eastern fringe of Berlin). In Rome, the remnant of the system comprises 3 isolated radial routes, not connecting in the ancient city centre, but linked by a ring route. Some apparently anomalous lines continue in operation where a new line would not on rational grounds be built, because it is much more costly to build a new line than continue operating an existing one. In some places, the opportunity is taken when roads are being repaved to lay tramlines (though without erecting overhead cables) even though no service is immediately planned: such is the case in Leipzigerstraße in Berlin, the Haarlemmer Houttuinen in Amsterdam, and Botermarkt in Ghent. Cross-border trams Tram systems operate across national borders in Basel (from Switzerland into France and Germany), Geneva (from Switzerland into France) and Strasbourg (from France into Germany). A planned line linking Hasselt (Belgium) with Maastricht (Netherlands) was cancelled in June 2022. Track Tramway track can have different rail profiles to accommodate the various operating environments of the vehicle. They may be embedded into concrete for street-running operation, or use standard ballasted track with railroad ties on high-speed sections. A more ecological solution is to embed tracks into grass turf, an approach known as green track. Tramway tracks use a grooved rail with a groove designed for tramway or railway track in pavement or grassed surfaces (grassed track or track in a lawn). The rail has the railhead on one side and the guard on the other. The guard provides accommodation for the flange. The guard carries no weight, but may act as a checkrail. Grooved rail was invented in 1852 by Alphonse Loubat, a French inventor who developed improvements in tram and rail equipment, and helped develop tram lines in New York City and Paris. The invention of grooved rail enabled tramways to be laid without causing a nuisance to other road users, except unsuspecting cyclists, who could get their wheels caught in the groove. The grooves may become filled with gravel and dirt (particularly if infrequently used or after a period of idleness) and need clearing from time to time, this being done by a "scrubber" tram. Failure to clear the grooves can lead to a bumpy ride for the passengers, damage to either wheel or rail and possibly derailing. In narrow situations double-track tram lines sometimes reduce to