Railroad ties are often used in many landscaping and garden projects. Most often steps and retaining walls. And while I don't use any in my own garden designs, I realize they are an easy to use do it yourself element worth mentioning.
Generally, as with these railroad tie garden steps, the ties are mainly used as retainers and borders that hold other elements as the tread of the steps. In this case, large cobble is used which is inexpensive, stable, and provides good drainage.