Note that when you click on a thread title in the "Show unread posts since last visit" page, you are forwarded to the last unread post. That's a bookmark. Check the URL; you will often see #new at the end. Unfortunately, that constitutes two navigation steps.
That wasn't the case until just recently.
My Railwire bookmark is to the "unread posts since last visit". I usually just click on the little "NEW" doohickey taking me to the most recent posts to the thread. When I hit the back button once, nothing happens (visibly), by hitting it a second time, I return to the unread posts, where I started.
This is only an issue on Safari for me, on my iPad or iPhone. I use Firefox on my PC and work laptop and am unaffected.