Due to an increase in traffic, Ferrocarriles Vascongados decided in the 1920s to electrify its main line between Bilbao and San Sebastián.

multiple unit types meant the 4000 series was increasingly used to haul local train services.

[2] The locomotive has a undercarriage that consists of two two-axle carriages with a cross-bearing drive, which are connected to each other with a close coupling.

On one side are the high-voltage cell, the cam controller, the vacuum pump for the train brake, and the compressor for the pantographs and whistle.

On the other are the low-voltage frame, the quick-action switch, the converter for the control power supply, the batteries, and the traction motor fan.

The cam gear is mechanically driven by the handwheels of the master controllers in the cabs via a chain transmission.