The series In consisted of 24 Atlantic express locomotives built by MÁVAG between 1906 and 1908. In contrast to the two-cylinder prototypes Il and Im built in 1901, they were four-cylinder compounds and were longer and heavier with a larger boiler. All four cylinders drove the first driving axle. Additionally, they were the first European locomotives to have a Vanderbilt tender.
In trials, one attained a speed of 142 km/h with a light train of 72 tonnes. With 406 tonnes, it could still attain 111 km/h. Due to their axle load of 16 tonnes, they could only be used on some lines. In 1911, they became series 203. 203.015 was rebuilt to a superheated simple, but remained the only one to be rebuilt. After World War I, some came to Romania and were used by the CFR.