Complex sentence: It used a top feed and contained 263 small and 50 large tubes plus superheater flues of diameter, while the round-topped firebox with its grate area was fired by a duplex mechanical stoker, the only class on the SAR which had this feature.
Split into simpler sentences: It used a top feed and contained 263 small tubes of outside diameter and 50 large tubes of outside diameter, while the superheater elements were of diameter. The round-topped firebox with its grate area was fired by a duplex mechanical stoker, the only class on the SAR which had this feature.