Here's a discussion that's pertinent and covers a number of focusing rail options: http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000Zuq

I use the Bogen 3419 rail and like it a lot. It's low profile, packs easily in the top of a camera bag, and has a threaded rod drive for fine focus (1mm/turn?) with a quick release for fast larger movements. I mostly use mine on the small geared Bogen head with a QR plate, which is really nice for "dialing in" the composition and focus in macro work.

Lee