If you are talking about the last one (since i changed prices and forgot which one is 50k), It is the last track piece, it spans around the entire screen like a big loop and finishes the track.
When I played it (just before I wrote the previous message), I unlocked the full loop when I unlocked the red area just after the blue one.
But there was still one last upgrade, which was located on the track and I guess that it was made to unlock the full loop (which I already unlocked).
So I guess it was a bug ?