Trains from Oruro to Uyuni? Wara Wara?

Hi there,

Has anyone taken the wara wara del sur train from Oruro to Uyuni.
As far as i´m aware of the Expresso del Sur leaves on Tuesdays and Fridays and the wara wara Wednesday and Sunday. Is this right?

I think my time frame means i may have to take the Wara Wara.
I read on another website that wara wara was a bit dodgy, does anyone have any experience of this?

Thanks