You line up by looking at the end of the box/rail (or where it would roughly be if you're approaching a box/rail where you can't see the end right away). I think you figured it out by the looks of your second post.
Where you look = where you go, so you line up by looking.
Also, just a tip for future posts, if you can use fullstops and paragraphs it'll make it a lot easier to read/answer your questions. It was really, really hard to understand your post because right now it's just a huge block of text without spaces.