It's a little late, but I would have mixed plaster, or used sculptamold, and filled in the bank and shaped it to cover the (tape?) under the tracks. Then, once it had dried, I would probably slather a cheap sealant paint over it, and then paint it with straight glue and sprinkle the sand product you show at present.
Anyway, you seem to be headed in the right direction, and it should look very good. I would say you need some duller green, almost more brownish, tufts if the proto photo is the standard.
It will be nice to see this when you call 'er dun.
-Crandell