Open the roller coaster file you have been working on.
Set your view mode to animation.
Here is what animation view is.
Right click the track so it is pink.
Click the down arrow next to object.
Select Path. 
When you click path - the speed of the roller coaster
appears. 
Here is what you are looking at.
On the X-axis you have the number of frames.
On the Y-axis you have a measurement of speed.
Press the tab key so you can edit the speed.
Right now the speed increases at the same amount all the
time. Right click on the the left side of the top bar with the three
vertices.
Press the G Key and move your mouse. Try to come up
with something like this. 
When the line is steep it means your roller car will move faster. The
the line is smooth the car with go at a stead speed.
Right Click the center vertex on the top.
Press the G Key and move your mouse to the right.
I moved my mouse to frame 375. This is about 15
seconds. 
Now I'm going to go back to where a hill on my track is.
I'm moving my roller cart using my right arrow key.
In the window for speed I'm going to press the I Key.
Then click Current Frame. 
Next advance to the top of the hill.
In the window for speed I'm going to press the I Key.
Then click current frame. 
Now zoom into your speed window.
To make the speed slower like a real roller coaster would be
I came up with a shape like this. 