Superheated steam and feedwater heaters are arguably two technologies that most increased the efficiency of railroad steam locomotives in the early twentieth century. These developments were the death of compound-steam locomotives.
Mark
Yes, what I've read is that the Belpaire was more efficient but was also more expensive to build. It was virtually a trademark for the Pennsy, frequently used by the Great Northern, but not in sigificant numbers on other North American railroads. This firebox was often seen in Great Britain.