History
SNCF had already experimented with three other dual-voltage locomotives in the 1950s, but these were all rebuilt from a DC locomotive and could only deliver their full power under 25 kV. Power output under 1.5 kV was reduced to 10% of the full power which is nearly neglectible. This was however no real problem, as these locomotives operated only on Aix-les-Bains - Annecy and only needed the 1.5kV DC to pull trains into the station of Aix-les-Bains or to drive to their depot in Chambéry.
In the late-1950s SNCF ordered the BB8500 and BB17000 locomotives. As all new locomotives were ordered in three versions at that time (DC-only, AC-only, dual-voltage), the same applied to the "Danseuses" locomotives: a dual-voltage version, called BB25500 was ordered later in the early 1960s but was part of the same locomotive family as BB8500 and BB17000 locomotives.
In order to get an idea of how to realise the first "real" dual-voltage locomotive, the damaged BB16540 locomotive was rebuilt in 1959 to dual-voltage and renumbered BB20004.
Read more about this topic: SNCF Class BB 20004
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