The E.551 was an electric locomotive that was used in the three phase network in Northern Italy. It was a direct development of the E.550, but was longer and heavier. A total of 183 were built between 1921 and 1925. Output was now 2,000 instead of 1,500 kW. They still had two traction motors mounted deep inside the frame which powered all five axles via coupling rods. By placing the traction motors in series or in parallel, a continuous speed of 25 or 50 km/h was achieved.

Although the top speed of 50 km/h suggested the use only in freight service, they also had a boiler to heat passenger cars. So they were also actually used in passenger service, as well as in helper and shunting service. Six were later retrofitted with multiple controls of the same system the E.554 had. The last one was retired in 1965, at a time when many lines had been converted from three-phase to DC. E.551.001 is the only survivor and can be found in the museum at Pietrarsa.