The first changes took place during World War II, following the destruction of 25 carriages during bombing raids. This total included the equivalent of three 4-RES units. A number of units were reformed to re-use the carriages that survived from individual units, and after the war was over new carriages were built to replace those destroyed, taking the numbers of the old ones. In total this involved eleven driving motor cars, seven Trailer Thirds, five Trailer Composites, and one buffet car and dining car apiece. This re-arrangement led to the loss of 4-RES units 3058, 3060 and 3063 and the formation of three new 4-COR units, which took numbers 3156–3158.