Why the 2*25 kV alternative? (autotransformer traction supply)

The author describes how it was the electrotechnical aspect in the form of reduction of losses on lines which was attractive in 2*25 kV traction power feeds in France; the first application was in 1981 on the new Paris-Lyon high speed railway line; this made it possible to avoid costly construction of high voltage lines to connect substations to the national grid distribution network. It soon emerged that as a result of the very rapid development of the public telephone network in France, the system was very useful to diminish interference on subscriber lines. The cost of the corresponding protection borne by the SNCF because of regulations in force accounted for an extremely high proportion of total electrification costs with the 1*25 kV system. At the end of 1992, in France, 12869 km of the track subgrade had been electrified. 6970 km were electrified to 25 kV with 107 substations, 1570 km of which were 2*25 kV with 25 substations and 5400 km of which were 1*25 kV with 82 substations.<
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