Clickable prototype: http://pauginer.github.io/prototypes/flow/responsive/index.html
The header for the side rail says: "About this discussion page", and has the X for collapsing the rail.
As the user scrolls down, the header anchors to the top of the window.
The header continues to appear even if the user has passed the end of the side rail content.
Clicking on "About this discussion page" is a way to scroll back to the top of the page.
The side rail can be collapsed even when it's further down the page.