Replacement of 3 rotted railroad ties in wall system
Given that your wall is six feet high, it is likely that there are perpendicular railroad ties (cross ties) in your wall. Since you can see three rotten ties on the face, there may well be rotten cross ties, which would be hidden from view. There is no easy way I can think of to determine how many cross ties are rotten, and there is no easy way to remove cross ties without taking down the wall.
If you end up removing the entire wall, you may want to consider rebuilding using segmental concrete block. I just built a small block wall in my backyard, it goes up very quickly, and the concrete will probably last a hundred years or more.