 Additionally, trailer cars (such as two 19XXs instead of a 18XX/19XX pair) have been irregularly placed as well.The final formation of two trains, trains 1-60~1-61, was a TC-M-M'-M-M'-M-M'-M-M'-TC formation (four pairs of M-M' car pairs with no trailer car pairs) that allowed for a slightly faster acceleration.When Seoul Subway Line 1 first opened, the Class 1000 cars were arranged in six-car trains, and were expanded to eight-car trains between 1980-1981 before being expanded to 10 cars by the 1990s.