Research and trials in Europe, particularly on French Railways (SNCF), indicated that 25 kV AC was an economical electrification system. Indian Railways decided in 1957 to adopt 25 kV AC as its standard, with SNCF their consultant in the early stages. The first 25 kV AC section was Raj Kharswan–Dongoaposi on the South Eastern Railway in 1960. The first 25 kV AC EMUs, for Kolkata suburban service, began service in September 1962. For continuity, the Howrah–Burdwan section of the Eastern Railway and the Madras Beach–Tambaram section of the Southern Railway were converted to 25 kV AC by 1968. Because of limitations in the DC traction system, a decision was made to convert the electric traction system of the Mumbai suburban rail network of WR and CR from 1.5kV DC to 25 kV AC in 1996–97. The conversion from DC to AC traction was completed in 2012 by the Western Railway, and in 2016 by the Central Railway. Since then, the entire electrified mainline rail network in India uses 25 kV AC, and DC traction is used only for metros and trams.