The most helpful thing in growing out a tail is to never touch it with a brush or comb. After you wash and condition it finger comb the tail, hair by hair. By the time you get through the entire tail,it should be dry or almost dry, then you can braid it and put it in a tail bag or tube sock. If you have a lot of flies or your horse tends to swish his tail a lot, I would suggest not using a tube sock, unless you are diligent in taking it down weekly, otherwise the excessive swishing of the tail creates friction which in turn will cause the tail to become a matted mess. Mtg or coconut oil is always helpful, but really the management of the tail is paramount.
I used MTG forever, and got nothing out of it except for a nasty, greasy tail.
For months now, I've been using water and coconut oil only. Daily, or as often as I can, I give the tail a hose down with water, keeping dirt from clogging the pores of the tail bone. Keeping it clean alone, with just water, keeps the natural oils flowing, protecting the hairs, and allowing growth.
Coconut oil, strengthens the tail immensely, and doesn't leave it greasy like most oils will. Since the hairs are softer, yet much stronger, there's less breakage that gives the appearance of a choppy tail.
It will take time, but with my water and coconut oil theme, my geldings tail is noticeably thicker, with a pretty little shine that it never had before.
When I do wash it, I use the human Tresseme Naturals shampoo (it's lower in sodium than most), very diluted, mainly just soapy water, and massage it in the whole tail.
I stay away from any regular products from the tack store. Most are high in silicone and sodium, which cosmetically make things look pretty, but really are of no help to the hair.
Nothing will actually make the hairs grow faster. What it does though, is prevents breakage, and allows natural oils to protect the hair, leaving things open at the root for the hair to grow easier.
I never stopped brushing my geldings tail, but I did do it very carefully. With a wide tooth comb, very slowly work from bottom to top, and no hairs are damaged. I section it out to three or four smaller sections as well, so I can brush it out better, causing less stress to the roots.