One way is to check “Fixed Position” in the Follow Path constraint panel. Then you animate the Offset value, which represents a fraction of position on the path; 0 = start, 1.0 = end of path.
Say the peak of your roller coaster is at exactly half the length of the path. In a 100-frame animation, you might keyframe Offset as follows:
0.0 at frame 0
0.5 at frame 80
1.0 at frame 100
The train would use 80% of the animation time to climb halfway up the track, and drop the rest of the way during the last 20% (ie, faster). You’d have to tweak these values as this example wouldn’t actually look realistic, but it gives the idea. You could even make the train stop or go backwards with this technique.