When the Aberdeen Harbour Commissioners finally agreed to allow steam locomotives to replace horses on the Aberdeen docks, the GNoSR purchased two 0-4-2T locomotives from Manning Wardle for the purpose. These were designated Class Y and numbered 114 and 115. On delivery these were found to be 2 tons over the maximum weight for operating on the docks, and a further two locomotives of similar design were acquired. These Class X locomotives, numbered 116 and 117, had smaller driving wheels and a shorter wheelbase. In order to reduce the weight of the Class Y locos, they initially operated with their tanks only partially filled.
Both classes remained almost unchanged throughout their working lives which involved working the 1.75-mile (2.82 km) long branch from Kittybrewster to Waterloo Docks in Aberdeen, local trips, and hire to nearby industry. They remained in the GNoSR livery of black with red and yellow lining until painted unlined black during World War 2, although they gained the letters LNER and subsequently acquired BR emblems. The locos also retained their original boilers, there being no spares. Except when on hire to industrial concerns, the locomotives were shedded at Kittybrewster.
The first loco to be withdrawn was 68193 in 1956, followed by 68191 in 1959. The remaining two locos were withdrawn in 1960.