They were the third variety of British Rail 's then-standard 1972 design for suburban EMUs , which eventually encompassed 755 vehicles and five Classes ( Class 313\/314\/315 \/ 507\/508 ) . They were the second variety of British Rail 's then-standard 1972 design for suburban EMUs , which eventually encompassed 755 vehicles and five Classes ( Class 313\/314\/315 \/ 507\/508 ) .