To replace pre-war railcars with a wooden underframe, the SJ ordered new vehicles in 1950. These were based on the narrow gauge YCo4p and YCo4t developed in the late forties and like these had gangway doors on the fronts. The first variant was designated YCo6, but later became better known as Y6. Despite a service weight of less than 20 tonnes, they had two two-axle bogies which ensured better running characteristics and a very low axle load. The power cars had a 145 kW (197 hp) diesel engine that transmitted its power to both axles of one bogie, resulting in a top speed of 115 km/h.

After the original variant that had 55 seats, the Y7 followed in 1957 with 47 seats for better passenger comfort. It took not long until the Y6 and Y7 had taken over almost all passenger service on branch lines. For even better comfort on some fast services, 29 Y6 were rebuilt to the Y8 between 1963 and 1965 where the number of seats was reduced to 36. Non-powered control cars and trailers were built in different variants, some only had mail and/or baggage compartments. In total, 373 power cars and 321 non-powered cars had been built by 1960. In the following years, some YS inspection vehicles were built on the same basis.

Electric variants with two traction motors and a total power of 170 kW (231 hp) were designated X16 and X17. While the first had 55 seats, the second had only 49. Although these only had a top speed of 110 km/h, they were also used on mainlines. The diesel variants began to be replaced by Y1 railcars in the eighties. Passenger traffic ended in the nineties.