Yes, interesting question.
Considering that shooting range berms\backstops are made of earth and do a great job of dissipating the energy of a fired bullet, I always treat something like a railroad berm as heavy cover.
Bearing that in mind, I would think that if your troops are hunkered down in a ravine or crater to where more than half of the models are obscured from the shooter’s point of view, I would consider it to be heavy cover.
This of course is determined as a prior agreement with your opponent.
Just my two cents – hope this helps.
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