People would not necessarily black out on a ride that tall. It would be a matter of the G-Forces. The G-forces would be determined by the track layout as well as the speed. I imagine that we are not all that far off from being able to build rides that tall. The trains could use a little more engineering, but it could happen.
With proper engineering the sky is the limit with rollercoasters as well as ferris wheels.
The one concern I would have is the bugs at that speed. I found the Gemini rather annoying at dusk. My sister's glass looked like a windshield with all the bugs stuck on them, and I couldn't open my mouth for a second or I would eat a bug.
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