I asked this question How does train travel work?
I went to check it out after getting another upvote on the question. I noticed that the comments were deleted. I notified someone who commented on the question, to make it an answer after some discussion.
Once a comment has an accepted answer, are the comments deleted?
Comments are meant to be for short temporary discussion, clarifications, or to add relevant but minor/transient information to the post (such as links to related questions).
The comments discussion was removed by a moderator once the info gleaned from the discussion had been posted as an answer. There was no need for the old comments to remain after that point.
You can read more on the article on Comments in the Help Center:
What are comments?
Comments are temporary "Post-It" notes left on a question or answer. They can be up-voted (but not down-voted) and flagged, but do not generate reputation. There's no revision history, and when they are deleted they're gone for good.
