Blowdowns and water chemistry:
Considering that I don't know the specifics of boiler water chemistry near the end of the steam era, I will go from what I have learned and was said on this thread. It was stated that caustic soda was used for pH control (keep it from becoming acidic) and sodium sulfite (dissolved oxygen removal). These are added to the water, either directly into the boiler or in the make-up water (storage tanks and/or water tender). These chemicals add dissolved solids to the boiler water.
As the boiler water becomes steam (for work - moving loco), the steam is essentially pure (free of dissolved solids). Since the solids are left in the boiler water, their concentrations become greater in the boiler water. More water is added to maintain safe boiler water level with more chemicals to regulate pH and remove dissolved oxygen. Over the length of the operation the boiler water develops greater concentrations of solids (sodium sulfites and sodium sulfates, plus whatever dissolved solids were already in the source water).
The longer the runs the greater the need to remove some of the solids (blowdown). The make-up water is less concentrated (solids) than the boiler water, so blowdowns would help extends run times between having to replace the boiler water.