I believe that the initial term used now (at least, I've seen it quite a lot recently) is "unexplained death".
I take that to mean that where there's no obvious cause of death straight away, it's called that until the autopsy reveals what the cause is. I'm happy to be corrected by those who know, though.
I may have read this wrongly, and apologies if so, but you appear to be conflating two different things. An 'unexplained death' means that the circumstances of the death (that aren't obviously at the hands of someone else) are unclear in terms of intent, ie was it suicide or accidental , and it will remain this way until the inquest when the Coroner makes his/her decision on the basis of the evidence presented.
The 'cause of death' just determines physically what caused the person to die, ie falling from height, multiple trauma from vehicular impact etc. That said, the phrases 'unexplained death' but with 'no suspicious circumstances' in a news story (see Tara Palmer-Tomkinson's death as a relevant and current example), tend to be police shorthand for them thinking drug overdose or something self-inflicted, though they may still be wrong about that. There still needs to be an autopsy to determine physical cause of death and an inquest to determine why the death occurred, but there's no suspicion that anyone else is involved.
In railway terms, if a person is hit on a level crossing of some description, then the cause is easy to define - multiple trauma due to vehicle impact, but the root cause will be less so without investigation. Was the person there intentionally or was it an accident? To compound this, and contrary to popular belief, the number of people who commit suicides and leave notes or an explanation is actually relatively low.
