The design was very successful and led to the development of the similar Class 320 and Class 322 units for use by Strathclyde PTE and Stansted Express ( now used by First ScotRail ) respectively . The design was successful and led to the development of the similar Class 320 and Class 322 units for use by Strathclyde PTE and Stansted Express ( now used by First ScotRail ) respectively .