This is the set-up I like best. Both a “tube” (for beginners) and “socks” (advanced) taped to the bar to better simulate the rail. (That’s what I think they have here.)

Dahlonega Gymnastics on Flickr – Photo Sharing!
I like the gymnasts to get off the tube as quickly as possible. They need to learn how to “flip” the wrists using the socks set-up as demonstated by the gymnast here. (Or is she using short pieces of tube?)
UPDATE: CoachL answered my question. This girl is actually using Ultimate Straps sold by TumblTrak.
Check an earlier post to see the socks taped to the bar.
Watanabe video – Working the Strap Bar
