The Harris trains were originally run as 7-carriage (M-T-T-M-BT-T-M) sets, reduced to 4-carriage (M-T-T-M) sets for off-peak and weekend services. The M-T-T-M sets were known as "Blocks", while the BT-T-M sets were called "Units". The first thirty 7-car sets were delivered between 1956 and 1959, and the following thirty sets between 1961 and 1967. Initially the two types were kept separate because of the differing motor unit performance, and within the second series the final five trains had to be kept separate because of the inter-carriage walkways. This proved difficult to manage and in 1962 the segregation instruction was withdrawn. The first carriage to have walkways retrofitted was 862T in 1967, when it was rebuilt following fire damage.