 When 1973 Stock was bought to re-equip the Piccadilly line, the three 1956 Stock trains and the 1959 Stock trains were transferred to the Northern line.They continued to work there until delivery of 1992 Stock began to replace the 1962 Stock working on the Central line.A few of the 1962 Stock trains were moved to the Northern line in 1993, to enable the 1956 Stock to be withdrawn, and to provide a modest increase in the number of trains running on the line.