Task: Question Generation
Input (English): Context : The Caledonian Railway 670 Class was a class of 0-4-2 steam locomotives designed by George Brittain for the Caledonian Railway (CR) and introduced in 1878. 
Subject : Caledonian Railway 670 Class 
Relation : service entry
Output (English): Which was the official year for the approval of Caledonian Railway 670 Class?