You have to set your detail sprite option in map properties to detail_trainyard.vbsp.(<thats an end-of-the-sentence-dot) Here is a link to all the other detail sprites https://wiki.teamfortress.com/wiki/List_of_detail_props
I would say its probably a nodraw brush with a grass texture on the side or a custom model, there a few now with the new inferno update that you can check out
It has to be a displacement with one of the specific blends that allow grass (I think they have an O in the top corner in the texture viewer)
You then use the paint alpha section of the displacement edit to "paint" where the grass grows.
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