Wash any surface dirt or debris off your railroad ties with water and a scrub brush. Railroad ties do not take pressure washing well, and soap is not usually necessary.
Allow the railroad ties to dry thoroughly.
Cut them to size using a chain saw -- watch out for old railroad spikes -- and place them in your garden or yard if they are to be used for landscaping.
Sand them down with a coarse-grit sandpaper on an orbital sander if you want to finish them further and use them as furniture or another indoor application.
Wipe them down with a tack cloth and then sand them to a finer finish with finer grits of sandpaper until they are the smoothness you desire.
Paint or finish the railroad ties with an oil-based paint or other finish you desire.