On Jul 16, 1:12*pm, The Natural Philosopher
wrote:

The need for pressure arises out of the need for efficiency so that a
locomotive can do reasonable distances carrying its own coal and water,
particularly water.

It is no accident that the high pressure "strong steam" engine was
developed in Cornwall. Oop north, the mines produced coal, so fuel
was plentiful and cheap, and making a more efficient pumping engine
was a low priority. In the Cornish tin mines, any fuel for the
engines had to be imported from eg south Wales, so fuel cost was much
more important to the economics of the mining operation. Hence, this
drove the move to more efficient engines, which allowed engines to
become compact enough to become mobile.

Robin