Detailed Instructions: Generate a question which can yield the answer mentioned in the input. Generated question must be answered by the answer provided in input, without using any extra knowledge.
Q: 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.
Answer : 1878
A:
Which was the official year for the approval of Caledonian Railway 670 Class?