Simplify the following text for a Grade 6 native English-speaking person.
Original: The British Rail Class 303 electric multiple units , also known as `` Blue Train '' units , were introduced in 1960 for the electrification of the North Clyde and the Cathcart Circle lines in Strathclyde .
Simplified: They are also known as `` Blue Train '' units . They were introduced in 1960 for the electrification of the North Clyde and the Cathcart Circle lines in Strathclyde .