I believe they are made of "carbon-fiber composite" according to the article. Apparently it does not stretch much.

I assume the problem of tension is solved by having a variety of belt sizes.

Anyhow, I think the point of this drive system is to use a bike specially designed for it.

Obviously it would be on the difficult side to upgrade a bike to use a belt drive ... and for this reason I think belt drive systems are a bad idea ... I am fine with owning one of those Trek Districts ... but on any other track frame it seems dubious at best.