Original: 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).
Simplified: 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).