Years ago when homes were sliding into the ocean in Pacifica, I heard that the natural erosion of the cliffs along the California coast averaged 6" per year (note that it is an average, some years nothing, other years a big slide). The people said when their houses were built they were set back 10 to 20 feet from the cliff. Well, simple math will tell you that your house has a little over 20 to 40 years before it's hanging over the cliff. A retaining wall mounted on the top of the slide won't do much. You have to put big rocks and riprap down at the base of the cliff to prevent the ocean from eroding the slope (like what the railroads did years ago).

The only real solution for the railroad is to do the rerouting away from the cliffs. As for the people and their houses, well, enjoy the view while you have it.