Yep, the back sure goes faster, going downhill that is. On the way down when the front hits point a, it will be traveling at a certain speed. When the back hits point a, it will be traveling faster because the train has accelerated due to gravity from the time the front hit point a. The exact opposite happens on the way up. Like Rideman said, This is most evident on the crests and valley's. Once gravity kicks in (after release from the lift), the train is going more downhill than up so the back will have a higher average speed.
On the Force though, I think The speed variation is less felt then lets say on the Gemini. It goes so darn fast wherever you sit!
By the Way, can anyone explain this better, or actually shorter. I try do explain to my friends that the back will be faster but they get lost, or they don't care.
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Joe E