Like this? RSS feed for replies? (Make sure your browser will display XML or open the URL in something that handles RSS feeds)
It’s just the URL to this topic with “.rss” added to the end. Make sure a post ID isn’t also on the end. So it should look something like ...topic-title/12345.rss, not ...topic-title/12345/2.rss
From looking at other topics with more than one (zero before I post this) reply, it looks like the RSS channel description is the first post and the items are a subset of the replies in reverse order.
That is, the first item in the RSS is the latest reply, followed by 20-something previous replies.
Update: Actually now that I’ve posted this and looked more closely at the feed for this topic, I see the opening post is also an item in the feed if it happens to be within the 20-something latest posts.