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as first, a rollback is when the train CAN'T make the top. so it actually Roll's backwards.
and as second, they slow you down at the top so you don't get too extreme g's at the top in the last car, but you hardly will notice it that you slow down...
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Incorrect, when the car does not excede the speed (120MPH) the car will got to the top and have a rollback... The car doesnt slow down it just freely falls backward. If you think about it the car would get more negative G's going up rather than during a rollback.
AND the black you see on the pulleys/ wheels it is just oil (quote some guy working there).