By the early 1960s, new stock was projected for SR main line operation, as most of the present main line units were nearing retiring age. During 1962/63, plans were drawn up for a new generation of electric multiple units, based on the external design of the CEP units. However, it was decided to introduce a break from the previous SR tradition. The driving vehicles were no longer to carry traction equipment or the guard's compartment, and all power and control equipment was to be housed in a Motor Brake Second (MBS), formed in the middle of a four car set. During 1962, orders were placed for Brighton line replacement stock, which consisted of 36 4CIG (four car Corridor Intermediate Guards) and 19 4BIG (four car Buffet intermediate Guards) units. Formations were: CIG — Driving Trailer Composite (DTC), Motor Brake Second (MBS), Trailer Second (TS), DTC, BIG these were the same as CIG units except for the TS car replaced by a Trailer Buffet (TRB). Another break from the past was that the build was carried out at York Works, who had a long history of electric multiple unit and coach building.
Each unit incorporated two DTC vehicles of slightly different layout. One was set out with three first and one second class compartment, a three bay second class saloon with two toilets at the inner end; the other vehicle had four first class compartments, a three bay second class saloon and two toilets. The MBS vehicles were mounted on two motor bogies, each axle having a 250hp traction motor, control equipment being mounted between the bogies. The internal layout consisted of a small luggage cage, guard's office and 6 1/2 second class seating bays. TS cars were basically a Mk. l locomotive-hauled TSO with accommodation for 72 in nine bays of eight. In BIG units, the buffet car provided 40 unclassified seats. The appearance of sets was similar to CEP/BEP builds, but incorporated a number of detail alterations, including a modified end gangway and the recessing of end pipe/control cables under the front windows. The first CIG unit, No. 7301, was constructed by York in 1963/4 and delivered in the autumn of 1964; the final unit of the initial build was delivered in early 1966. The BIG fleet, numbered 7031- 48, was not delivered until late 1965.
In the closing years of the 1960s, new stock was required for the Waterloo to Portsmouth route and a further order for fleets of CIG/BIG units was placed. These were of the same internal layout as previous builds, except for a more modern decor, together with some technical refinements. Unit numbers for Portsmouth line stock were 7337-66 for CIG units, and 7049-58 for BIG units. In 1969 a further order for 71 CIG units was placed and allocated numbers 7367-7437, delivery commenced in 1970 with allocation to the South Western and Central divisions to replace COR and other ageing stock. Before the end of the CIG production run, a further unit, No. 7438, was ordered as a replacement for a withdrawn CEP unit.
The CIG/BIG fleets gradually took over London to South Coast duties which were previously operated by COR/BUF/GRI/PAN and PUL stock, on both the Central and South Western divisions, being allocated to Brighton and Fratton depots. Under the TOPS system, CIG units were classified as 421 and BIG units 420. The original units for the Brighton line were outshopped in green, whereas later units emerged painted in InterCity blue/grey livery.
By the 1990s a major facelift operation commenced on the CIG fleet, this extended to the BIG fleet, which saw some reformations of sets. After buffet facilities were withdrawn from the waterloo-Portsmouth line the remaining BIG sets operated for some time with the buffet cars isolated and eventually were rebuilt as CIG sets with spare Class 411 TS cars replacing the buffet vehicles.
In 1997-98 a batch of 11 sets were modified to 3-car formation by removing the TS and at the same time opening out the driving cars to all saloon layout, these sets were re-classified as COP (Coastway OPen) and used on the Brighton-Coastway services.
In the late 1980s some short term reformations to include buffet cars were made on the Brighton line.

12ft 9 1/4in (3.89m)
(ex CEP vehicles 12ft 6in (3.81m))

TS - 31.5 tonnes
(Ex CEP vehicles 33.8 tonnes)