Bob
The Titebond III will be good .. you can easily break it up if need be ,
Titebond when soaked in is a little flexible ( use some dish soap , and mix 50/50 ) you can adjust the mix to your liking..
I usually will do this on the outside of the gravel to hold the loos middle gravel in place, like a wall, letting the gravel drain thru the bottom and let the track move or expand .
The Portland cement forms a hard skin ( if you sprinkle it on top, or hard as a rock when mixed in with grout)
Looking at your first video's cover shot, I see rails ..I do not think you need the Titebond III to keep it in place... You did incorporate water letter outers... didn't you ...
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