4) Yet the one you see here drawing the Royal Scot past the water troughs at Bushey, near London, is the logical development of that old colliery locomotive.

You probably know that trains can pick up water while travelling, from troughs such as these, to avoid delay. But in many cases the water has to be specially softened, for hard water such as you get out of the tap in some districts would fur up the boiler like the inside of a kettle, and quickly make it useless. In some districts, however, the mains water is soft enough to be fed directly to the troughs.