Hi,
I have some problem with particle collisions when I start the game. Otherwise Everything seems to be okay.
Terrain have a box collider.
When I simulate it, then it works fine but when i start game then particles goes through floor 
In your Particle System, in Collision area, there is a setting which tells particles with what layers to interact. In your case, it’s “World” layer. Try setting “All” to have particles interact with everything.
Otherwise, make sure that terrain is in World layer.