For those of you who are interesting in buying a good focusing rail for macro shooting and, in particular, focus stacking, here's a good video: https://lensvid.com/gear/choosing-the-best-focusing-rail-for-macro-photo.... (Thanks to John Hulburt for discovering it.)
Much of the front end of this video goes into focus stacking, with some details that you may or may might not find useful. For example, the narrator maintains that the best way to focus through your subject is by using a lens fixed in place on a bellows, a device that hardly anyone owns or uses. His second recommendation is a lens with a very long "lens throw," which also is hard to find and requires focusing with the focus ring on the lens--something that most other people do not recommend. His third recommendation is focusing with a rail, which most experienced stackers recommend as the best way to go.
Another thing you'll note is that the narrator uses Zyrene Stacker rather than Helicon Focus. I've used both of these products and have been recommending HF as the best of the two. However, this video prompted me to look at the latest testimonials on these two products, and it seems there is a lot of praise for the current iterations of ZS. Therefore, before you buy either of these products, I recommend that you download each one as a trial and test them yourselves. If you like ZS better, then you'll be spending a lot less money!
The video begins discussing focusing rails at about 22 minutes in, and that's definitely worth watching. The top choice for a currently manufactured rail is the Novoflex Castel-L, which sells for $300 at Ace Photo online and for $311 at B&H and elsewhere. (His other favorites haven't been made for years and are only rarely available on eBay or in camera store used departments.) The only fault with the video is that it does not review the Kirk Enterprises rail (FR-2), which also might be an excellent choice. (I have the FR-1, which I like very much but which is no longer being made.)
After considerable research comparing the Kirk, Novoflex, and Hejnar high end focusing rails, I very recently purchased a Hejnar 8" MS3_8 rail which is Arca-Swiss compatible. This is currently on sale for $300, and you can get an extra 5% off using the coupon code: SCV2016-2. All these rails have worm screw drives that are capable of very fine adjustments. Besides having probably the best reviews, this Hejnar rail has an overall length of 8", or 203 cm, which will allow stacking deeper objects. One can buy the Hejnar rail from the Hejnar Photo online store. As of the beginning of next week, when I expect to receive my new rail, I will have two other rails which I no longer need that I was planning to sell on Ebay, but I'll be happy to sell them locally. The rail I've been using most is a Velbon super mag slider which retails new at Anorama for $129. This is a worm drive rail, and is considered the best of the medium priced rail. I paid ~$65 for it used about 6 months ago on Ebay. It's in great shape, and I would be selling it for the $65 I paid. The second rail is a Spiratone rail, which is one of the good medium priced original rails. Spiratone, a Japanese company, is no longer in business. This design is the most popular design copied by the Chinese sellers on Ebay. The big differences is that the original Spiratone is all metal, while the Chinese copies are plastic. The Spiratone has a rack and pinion metal drive. My asking price is $15. Let me know if you are interested. sdsilverst@gmail.com
I just purchased the new Kirk FR-2. It is excellent quality and should work very well. Note that at Kirk Enterprises website under Outlet, there are blemished units available periodically at a lower price.