The six members of the H2 class were a superheated development of the H1 Marsh Atlantics, authorised in April 1911 and entering traffic between June 1911 and January 1912, with minor alterations to Marsh's original design by Billinton. The prototype 4-6-2 J tank locomotive, no. 325, was completed in December 1910; construction of the second engine, no. 326, started in May 1911. Since a number of problems were being experienced with no. 325 in traffic Billinton was given permission to halt work on no. 326 in June to concentrate on repairs to existing stock and construction of the H2 class. By the beginning of 1912, when construction recommenced, Billinton had modified the design sufficiently for the locomotive to be given the class designation J2 (the prototype becoming J1). Modifications included replacement of the Stephenson valve gear with Walschaerts gear, a reduction in water tank capacity and the use of spiral springs in the suspension instead of leaf springs. Although no. 326 had better acceleration and a faster top speed than no. 325 no further members of the class were built, in part because the 10 members of the superheated I3 4-4-2 class built between August 1912 and March 1913 proved to be the equal of the J2 class in performance and reliability.