 However, the extra length of these locomotives (they were a total long) made them unsuitable for a number of branch lines where only a turntable was available.The J class adopted an alternative approach to the problem by utilising a high-set boiler (with the boiler centre above rail level, compared with for the K class) setting the firebox above the frames and driving wheels, and retaining the K class' short wheelbase.The J class also featured a number of other design advances over the K class.