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