This is cross posted in Live Steam to get the widest spread of answers.
Aboard Navy steam ships, and I suppose steam ships in general, boiler feed water is distilled to a higher purity that is potable water. This is to protect the boiler from impurities in the water forming scale on the “pipes” that would hasten the breakdown of the boiler.
Yet, water purity does not seem to be a concern with steam locomotives. It was not uncommon for a locomotive to draw water directly from a river. It might go temporarily into a tank, but there was no processing of the water that I know of.
Why was there a difference between boiler feed water at sea and on railroads.
SteveF