ATM will finance the €109m order from its own budget and deliveries will take place between the end of next year and August 2018, reducing the average age of rolling stock on Line M2 from 27 to 16 years.
The option is from an initial €220m order placed with AnsaldoBreda in 2012 for 30 Leonardo trains, which comprised 20 trains for Line M1 and 10 trains for Line M2.
The 107m-long six-car trains accommodate up to 1232 passengers, 204 of them seated, and have a maximum speed of 90km/h.
The trains were designed by Italian companies A+B, Dominoni and Quaquaro and assembly will take place at Hitachi Rail Italy's Pistoia plant.
