Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I am not an expert on the properties of steam, but steam does continue to work down stream of the boiler.  If you look at a compound Steam engine of lets say three cylinders,  the steam from the boiler goes into the smallest diameter cylinder, and is exhausted from there into the next larger cylinder and is exhausted from there into the largest diameter cylinder.  The steam continues to expand as it cools, but the pressure decreases which is why the diameter of the cylinder increases.  I believe you are correct in that the highest pressure is at the output of the boiler, but since the properties of steam are such that it continues to expand as it cools, it continues to work.  This is somewhat different then what compressed air does because compressed air does not continue to expand.