To get technical about it, you have to make a displacement and paint the alpha on it. It's usually a blend texture, so filter for blend when you look at the textures. Some of the textures say "skybox" or "nodetail" those are versions of the same texture minus the detail props (for optimization purposes)
It should show up in hammer when you make your alpha blend. So it should be obvious when you manage it
But you'll need to change your detail sprites in your "map properties" if you want green grass rather than brown.
By default it is set as "detail/detailsprites" which displays the dead (brown) grass. Change it to "detail/detailsprites_trainyard" and also change "detail.vbsp" to "detail_trainyard.vbsp" for green grass.
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