Boiler Capacity – The haulage power of a locomotive depends largely upon the capacity of the boiler. The prevention of steam condensation at the cylinders, combined with an increase in volume of highly superheated steam is significant to an increase in boiler capacity; in other words, with a given boiler size its capacity is equivalent to an increase in tractive power.

Compounding – By the use of highly superheated steam, compounding in many cases could be dispensed with. This made the locomotive simpler to a large degree. Compound locomotives using superheated steam showed very little superiority in economies compared with simple engines similarly fitted. The more recent history showed that two or four cylinder engines working with moderate steam pressures of 140 to 170 psig (10 to 12 bar) slowly replaced compound engines.

Increase in Haulage Power – The simple locomotive with a Schmidt superheater consumes about 15 % less steam than a compound engine with the same boiler but using saturated steam, which equates to

power increase. At ordinary speeds of express trains, only 60 % of the power is transmitted to the draw-bar, the balance is absorbed by the engine and by the tender resistance. Consequently, an increase in power of 17.6 % represents an increase in haulage power of:

in favour of the simple engine. These numbers were based on actual tests. At higher speeds the difference between the two concepts is even more important. The difference can be as high as 66 %. There is an accumulating effect caused by smaller coal and water consumption, which resulted in smaller tanks and tenders.

The two figures below demonstrate the differences graphically.