This practice is used for funiculars with slopes below 6%, funiculars using sledges instead of cars, or any other case where it is not ensured that the descending car is always able to pull out the cable from the pulley in the station on the top of the incline. Another reason for a bottom cable is that the cable supporting the lower car at the extent of its travel will potentially weigh several tons, whereas that supporting the upper car weighs virtually nothing. The lower cable adds an equal amount of cable weight to the upper car while deducting the same weight from the lower, thereby keeping the cars in equilibrium. A number of cable railway systems which pull their cars on inclined slopes were built since the 1820s. In the second half of the 19th century the design of a funicular as a transit system emerged. It was especially attractive in comparison with the other systems of the time as counterbalancing of the cars was deemed to be a cost-cutting solution. The first line of the Funiculars of Lyon () opened in 1862, followed by other lines in 1878, 1891 and 1900. The Budapest Castle Hill Funicular was built in 1868–69, with the first test run on 23 October 1869. The oldest funicular railway operating in Britain dates from 1875 and is in Scarborough, North Yorkshire. In Istanbul, Turkey, the Tünel has been in continuous operation since 1875 and is both the first underground funicular and the second-oldest underground railway. It remained powered by a steam engine up until it was taken for renovation in 1968. Until the end of the 1870s, the four-rail parallel-track funicular was the normal configuration. Carl Roman Abt developed the Abt Switch allowing the two-rail layout, which was used for the first time in 1879 when the Giessbach Funicular opened in Switzerland. In the United States, the first funicular to use a two-rail layout was the Telegraph Hill Railroad in San Francisco, which was in operation from 1884 until 1886. The Mount Lowe Railway in Altadena, California, was the first mountain railway in the United States to use the three-rail layout. Three- and two-rail layouts considerably reduced the space required for building a funicular, reducing grading costs on mountain slopes and property costs for urban funiculars. These layouts enabled a funicular boom in the latter half of the 19th century. In 1880 the funicular of Mount Vesuvius inspired the Italian popular song "Funiculì, Funiculà".