As far as I know (and this is all from information I've heard through others), you pay $10 for five FastLane passes. They can be used on any of the five specified coasters (X-Flight, Villain, Batman: Knight Flight, Superman: Ultimate Escape, and Serial Thriller, I believe). You can use them any way you wish. Meaning, if you want five laps on X-Flight, you can use all five there. If you want one lap on each, you can do that too. If you want four on S:UE and one on Villain, go for it.
Different rides, though, have different procedures for using FastLane. From what I've heard, some coasters have specific rows arranged for FastLaners, while others just board anywhere on the train.
Again... I have
not used FastLane nor witnessed it first-hand. This is information I've gathered about it over the season.
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Matt Lynch
Co-Webmaster, Kennywood Boulevard
http://kennywood.coasterbuzz.com