Hmmm, let's see here...48 feet across, in 2 sections, separated by 15 steps up to your parking area. Say about 9-10 feet high, and say three cars in the lot...
That sounds to me like a lot of weight to hold back and IMO' - just given the height - railroad ties would not have been my first choice, given that, the creosote content of the ties (or something similar) and the proximity of the lake. Off the top of my head, I'd say a 9' high retaining wall with just grass above it, would have needed a 60 degree angle and therefore would have had to go back into the soil at least
6 feet. Add extra weight to that and to my mind, this is a temporary structure at best.
I would have rather seen a proper concrete wall, proper drainage tube, geotextile and interlocking stones in the first place -and certainly as a replacement. Hopefully that's in your budget.