Suppose that you and two classmates are discussing the design of a roller coaster. One classmate says that each summit must be lower than the preceding one. Your other classmate says this is nonsense, for as long as the first one is the highest, it doesn't matter what height the others are. What do you say

The second classmate is right.

Explanation:

The height of first summit provides the potential energy it will use to climb the following ones.

Ep = m * g * h

Where

m: mass

g: acceleration of gravity

h: height

When the train goes downwards the potential energy is converted into kinetic energy (manifested as speed) and when it climbs it consumes its kinetical energy. As long as no summit is taller than the first the train should have enough energy to climb them.

Also it must be noted that friction also consumes energy, and if the track is too lomg all the energy might be consumed by it.