Can't see your pics.
Beat me to it.
If the drop isn’t any more than 3 feet, get some (hand picked and clean) RR ties to build the retainer wall. Make sure there’s no hollow spots in them. Notch them where they meet in the corner, and make sure you stack them overlapping like bricks. A piece of 1 inch pipe driven at least 24 inches into the ground (4’ long total) on 24 inch center (actually 1’ from the ends and 3’3” for the two middle holes) will give you well built wall. Put a couple inches of 57 under them for drainage, and backfill with 57. Don’t build too close because this is to stop erosion, not support a load.
You will destroy ANY tool you use to make holes in or cut ties. Stone chips get embedded in every check and split, and you know they are there when you see the sparks or the tool stops cutting. Get good carbide drill bits for drilling, and figure on replacing your saw chain. Auger type bits work best in gummy creosote, but don’t even think about standard augers for wood boring. Did I mention getting a good supply of files for your chain saw and extra chains? I use the little cheap sharpener from HF to correct the worst damage, then file the edge to sharpen it for the next cut.
A little secret I learned about ties is to set them upside down so the side that has been ‘finished’ by the gravel in the rail bed is the side that’s seen and a smoother working surface for setting the wall. This also hides the spike holes and wear plate wear as well as reducing the likelihood of nasty splinters down the road.
Otherwise, dig and pour a footing, build the wall with block, fill the cells with concrete, and make sure to seal the top with cap block. Not quite as‘natural’ looking, but if it’s the back side, who’s gonna see it. Block walls are ugly, IMO.
Ties are cheaper, but harder to handle and process for the construction, but carefully chosen ties will likely last a lifetime. A side benefit is the creosote will keep a lot of bugs away. If they can dry back out between rains, they’re the gold standard for treated wood. Don’t expect them to be straight or perfectly sized.
Cinder block and concrete is even more permanent, but doesn’t lend itself to possible future modifications or expansion. Make sure you’re not gonna be wishing you put the wall in a different place.
Plan C is to use the block retainer wall as the foundation for the back wall.