I don’t think the track needs to be circular.
If it doesn’t you could imagine running it between cities or destinations and also allowing other trains to run on it during peak hours.
If you could remove overhead lines from (disused) regular train tracks and instead use some other method to transfer electricity to and from trains and this concept train, that would be good. If you had a single track rail, perhaps you could use passing loops to allow other trains to pass, allowing you to use \frac{1}{\pi} length of track compared to using a circle.
I guess it could work anywhere where regular rail also mostly works. I think you’d need to know the efficiency of regen to know at what inclinations it stops making sense.
In trains that is done using overhead lines. I don’t see why something like that, but not directly overhead, couldn’t be done here.
I don’t know if there is more testing that was done beyond this, some while ago: https://www.youtube.com/@X-windDe/videos you would think the train and kite would need some development that you could do before you built or modified more than a short train track.
Perhaps there are still some regions where train tracks don’t have overhead lines, you could test your system there (now with battery storage).
The US railway system is not electrified and Europe is still only at 60 percent it seems:
You can make your own train track map across the US say and schedule your trains according to the weather forecast. You’d need to add a discharging station at every stop.
ChatGPT on how to get a new concept and trains on the US rail network: Slow Chat - #783 by Windy_Skies