The class were numbered 677–686, and were a development of the X2 class, based on experience gained with the locomotives in traffic. The boiler was based on that used in the T3 class, and shared the main dimensions.
Table of locomotive orders
All passed to the Southern Railway at the grouping in 1923. Withdrawals started in 1933, and by the end of 1937 only two remained. No. 684 went in 1940, and the last, 681 was retired in April 1943. All were scrapped.