Hi Leo,
A short answer to start with: You can compress Gas
(steam) but you cannot compress liquid (water)
After a loco stands
for a while, any steam still in the cylinders cools and condenses back to
water. When starting the engineer will open drain cocks on the cylinders
to let the condensed water out then close them to keep the steam in to do
its work.
Another thing that happens and what may have happened
here is that the boiler was most likely full of water,and as they tipped
over the hill and drifted down grade, the water in the boiler entered the
steam dome (where the throttle valve is at) and when Eng Nelson opened the
throttle instead of admitting steam to the cylinders, it let water in and
as water cannot be compressed the mass of the engine and momentum, kept
the side rods moving, even though the pistons for all intense purposes
"stopped"....and "poof" bent rods.
Any others with further steam
experience, please chime in.......................
Hope this helps
Leo.
Greg K