The process of preservation was called the "Reaping Process".
The bark was stripped off and the poles put into big cylindrical iron
tubes
and the tubes were then sealed.

You omitted to explain that the chamber was then sucked of air to form a
vacuum, which of course sucked out air from the timber as well, so that when
the creosote was sprayed in, it penetrated the poles as well.

City & Guilds Line Plant Practice "A"

Mike
another one of "God's Poor Orphans" now well and truly retired ;-))