The size was constrained by the heavier axle - loading of Maunsell 's proposed 2 - 6 - 4 tank locomotive variant of the N class , the K class .
Was consequently smaller than was otherwise possible on the 2 - 6 - 0 chassis .
The size was constrained by the heavier axle - loading of Maunsell 's proposed 2 - 6 - 4 tank locomotive variant of the N class , the K class , and was consequently smaller than was otherwise possible on the 2 - 6 - 0 chassis .