On their 27 mile test track. Which is too bad since at 300mph that would be a 4 minute ride :-) I expect it spends all of its time accelerating and decelerating. I suspect such trains will only be practical in the 'real' world by having them in some sort of fully enclosed track. It doesn't have to be a solid tunnel but something that keeps out birds and other debris from the area. Nothing like hitting a pigeon or catching a flying bit of branch at 300MPH, hurricanes put 2 x 4s through houses 2/3rds that speed so that would be a challenge.

That said the only other remarkable thing was how shaky the video was at 500km/h if the train was really moving around that much it wouldn't be too sustainable. One of the amazing things for me on the TGV was how smoothly it rode even going 200km/h.