A design for a small multi-service Bo-Bo locomotive, with speed up to 90 km/h (56 mph), had been devised by Giuseppe Bianchi as early as in the 1930s, but the project had been halted by the introduction of the E.326 and E.428. After the realization of the 6-axle E.636, it was however decided to revamp the project by adapting to it some solutions already in use on the E.636 (engines, bogies, suspension etc.).
Breda SpA workshop provided the three prototypes of E.424 in 1943-1944. Mass production, however, could begin only after the end of World War II, part of the funds provided by the United Nations Relief and Rehabilitation Administration program.
The locomotives have been built in four series starting from year 1943:
Class E.424, having DC type motors like all the Italian locomotives of the time, is a rheostatic-type locomotive; on start, a rheostat is connected in series to the motors and is gradually excluded as speed builds up allowing more current to flow to the motors; unlike other rheostatic locomotives of that time, this is not achieved via the characteristic lever (Maniglione), but through an automatic system, called Avviatore Automatico, derived from contemporary first-generation ALe 790/880/883 EMUs.
The driver simply selects the combination (series or parallel), and the relative rheostat contactors are automatically and gradually closed by this system; in case of failure, the driver can manually rotate an apposite wheel (that also usually rotates automatically as the system advances) to proceed with the exclusion.
When the rheostat is completely excluded for the series combination, field shunts can be inserted, or the driver can proceed to parallel combination, making a transition, which is handled by a device called "CEM" that automatically combines the motors (closing various contacts) accordingly.
Originally more than one unit could be coupled and be controlled remotely by the first locomotive using a system called Comando Multiplo; on the central part of the cab there also was a door that enabled the crew to pass from one locomotive to another in case of problems. However, because of safety issues and the imperfect reliability of this system (there were no instruments indicating the status of the slave locomotive, so it was not certain if it correctly made a transition or not, for example), it was abandoned and then disassembled.
Eleven units received compound-type motors, enabling a very fine speed control, in a range included between 25 and 50 km/h (16 and 31 mph) in series combination, and 50–100 km/h (31–62 mph) in parallel.
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