Repair or rebuild your retaining wall
If a wall is leaning, bulging or cracked through, the problem is usually behind it or under it, so patching the face won't hold and I'll tell you a rebuild is the honest answer. If it's standing straight and the trouble is surface wear, a repair makes more sense. For old timber or railroad tie walls, I'd go with block, because the masonry industry's design manual says block units resist cracking and don't splinter or decay the way treated wood does.

