I’m not that well up in mechanics myself, but I can tell you that I took this photo in a steam loco which was up in my local railway station. The lubricator works by steam entering the chamber, cooling and sinking to the bottom. This in turn raises the level of the oil which is then carried through pipes to lubricator moving parts of the engine. This is very much a laymans explaination which is about the extent of my knowledge on the subject. For further information I would suggest that you Google the term.