From last night's CNN article on the whole Spanish high-speed train story, it appears that the new high-speed rails were laid in old roadbed, and to save money they didn't increase the diameter of each turn radius to allow for faster turns.
So these express trains were speed-restricted in turns, only able to go fast in the straightaways, and forced to slow down when turning.
They were forced by two things: posted railroad orders, and the laws of physics.
It appears our intrepid engineer thought he could break both rules. Better luck next time!