After the First World War, the Helsinki suburban traffic, serving ever expanding areas of population, became too heavy for the various I class 2–6–4Ts and an entirely new type of tank engine was designed. The first plans were for a larger 2–6–4T but following discussions with Hanomag the decision fell on a 2–8–2T for suburban traffic and a similar 0–10–0T for heavy shunting. Orders were placed in 1923 and 1924 for the purchase of 16 2–8–2Ts, eight from Hanomag, two from Tampella and six from Lokomo. The Hanomag engines entered service in the autumn of 1924 and the Finnish-built ones mainly in the winter of 1925–26. They were classified N1 and given the running numbers 761 to 776. In 1942 they were reclassified Pr1.
The new 2–8–2Ts had an ordinary pivoted leading truck and a Krauss-Helmholtz -type trailing truck. Superheated, these simple expansion locomotives had bar frames, Walschaerts valve gear using piston valves, side water tanks mounted on the boiler sides above the running plate, a Knorr feed-water heater, and Aga lighting. Their maximum permitted speed was 80km/h in both directions.
The originally fitted Knorr feedwater heater was replaced by a Friedmann live steam injector between 1935 and 1948, ten of the 2–8–2Ts being so modified in the pre-war years, the remainder between 1945 and 1948. Electric lighting was fitted between 1948 and 1950. The N1/Pr1s worked as wood-burners during the Second World War but tests using oil as fuel led to the decision to convert the whole class to oil firing in the immediate post-war years when oil prices were favourable. Time permitted only the conversion of nos. 765–766 and 771–773 between 1947 and 1948 before the cost of oil began to rise substantially, resulting in the abandonment of the project and reversion of the five oil-fired engines to coal firing between 1950 and 1951. A shunter’s step was attached below both buffer beams to aid shunting staff in 1968 and 1969. Certain Pr1s were also equipped with Müller pressure equalising piston valves in the late 1960s.
All sixteen engines were allocated to the suburban services in the Helsinki area, duties upon which they reliably performed for over forty years. All trains as far as Järvenpää and Siuntio were hauled by the N1/Pr1s, longer turns being prevented by their limited water capacity. However, these tank engines were regularly used for stopping services as far as Riihimäki and Karjaa. The Pr1 tank engines worked the Helsinki–Turku expresses between Helsinki and Kauklahti during the Russian occupation of the Porkkala Peninsula between 1947 and 1956. They were also used to haul freight trains from Helsinki to Riihimäki or Karjaa as a result of their remarkable drawbar power, working such turns in the evenings or at night once the busy suburban passenger traffic had eased. In practice this meant that Helsinki never needed a large allocation of dedicated freight engines as long as the N1/Pr1s were available. Their rosters also included yard pilot duties and trip workings between the various Helsinki marshalling yards and harbours.
Diesel railcars took over the majority of the Pr1-worked Helsinki suburban services in the late 1950s, leaving only the peak period trains locomotive hauled. Subsequently, the Helsinki Pr1 stud was partially diminished by transferring nos. 761 and 762 to Kouvola and Riihimäki respectively. The Pr1 tank engines were then gradually replaced in the mid-1960s after Sr12 class diesel-hydraulics nos. 2701–2712 were allocated to Helsinki and with the general increase in diesel motive power. Nos. 763–766 were stabled at Tampere between 1965 and 1968 for the daily pair of Toijala return workings and an Orivesi local turn. Pr1-hauled local services came to an end at Tampere in late 1967 and at Helsinki in the spring of 1968 respectively but the class remained active as shunting engines for some years, mainly in Riihimäki, Kouvola, Pieksämäki and Joensuu.