Boiler pressure processes have been successfully used to impregnate railway sleepers for more than 150 years: A simple Rüping process is used for oak, pine, and larch sleepers. The improved double ruffling process is used for beech sleepers, and the boiler vacuum pressure process is used for pine sleepers.
These processes differ in the process parameters (height and length of the individual vacuum and pressure phases) and types of protective agents (the two Rüping processes use oil preservatives, and the boiler vacuum pressure process uses aqueous wood preservatives).