Energy conservation

[Yaqout]

Assuming the roller coaster is towed to the top of the hill on the left-hand side of the diagram and released. How would the conservation of energy work. I know that at the top of the first hill there would be P.E. and as it gradually rolls down the K.E. takes over. But how about as it progresses up and down the track. I'd like to know how can it keep conserving energy throughout the whole ride.[?]

To see the diagram talked about click the following:
http://www.geocities.com/h_mussa77/rollercoaster.gif

[HallsofIvy]

That is, by the way, as long as the car is rolling freely. Of course, on a real roller coaster, work is done to raise the car up to the top which adds energy and there is some friction which subtracts energy.