Tubular rail is cheap to build, but stupidly expensive to maintain and comes with high risk.
Imagine disengaging and reengaging a set of wheels every 30 meters and having that engagement to be so precise as to provide enough friction to further push the train, but not enough that you get a heavy thump at every one.
The wear caused by that engagement is significant, and over time, one or the other component will fail, or send it off-course to the next one. And while the first one may only be off by 5mm, the next one might be off by 10cm as a result of that. And a 10cm impact of a train at 240km/h isn't going to be a small bump.
Way to talk about a concept like it's a mature and commonly understood technology.
It seems like the rings are going to have to be precise enough that the train rides smoothly or not at all. From the look of it, the train itself is a completely rigid structure with a length spanning 3 or 4 of the rings, it's not going to be able to ever get 5 or 10cm off course.
Just like with today's rail the rings would obviously need to be regularly maintained. I'm sure there are very cost effective sensors out there that you could install into each tower to detect movement or misalignment. Hell, depending how much you want to spend on fail safes, you could even have lasers shone along the tracks to ensure perfectly straight wheels through minor on-the-fly mechanical adjustments.