[nitpick]Ok, this is making me sick. We can put men on the moon but we can't put a loop on vertical track? Pleeeease![/nitpick]
Now for my solution. I know it's hard to visualize, but try your hardest. In order to get the train up and down through a loop safely, have a somewhat deformed loop. Make a wider turning radius at the top of the loop, and a smaller radius for the bottom part. Just imagine a "Large Half Loop" in RCT then a regular "Half Loop" followed by another slope up to vertical. Since you would have greater speed coming from the launch, there is a larger radius at the top. That should slow you down enough to bear the smaller radius at the bottom of the loop. Once you are thru the loop and into the vertical spike, you lose speed, drop, and only gain a little bit of momentum before entering the bottom, smaller radius portion of the loop. With your decreased speed, it would take less energy to get thru the small loop half than the larger part. Give me about 5 minutes and I'll upload a drawing...
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Six Flags America? No, no! My name means "Sarcasm-For-All"