With the Class 502 units now life-expired, unable to cope with the demands of the new "Link" tunnel and approaching 40 years old, by 1977 a replacement was sought. Owing to the success of the Class 313 fleet on suburban services from Kings Cross, two sets were temporarily transferred to Merseyside and based at Hall Road TMD. Sets 313003/013 were used for over 250 miles of extensive testing on the Southport, Ormskirk and Kirkby-Garston lines. The results revealed a similar version of PEP-derived stock would be suitable for the Merseyrail Northern Line. Initially, 47 sets were ordered (507001-507047) but cost issues forced this number to be reduced to 38 units, then as low as 30 by early 1978 when the first sets were under construction. Ultimately, 33 units were built between 1978 and Octobe