It took about a week to build (I didn't have much time to build),
and an estimate of about 800 - 1100 pieces.
The mechanism to pull the cart to the top is, obviously, the work of a pulley with strings.
The release mechanism is a simple locking and unlocking gear mechanism that is activated with a little push and the cart races back down from its own gravity.
Which is why I named it Gravity.
Step 1: The Base
You don't need the exact color, but be sure to build it exactly. Put the motor where it shows.
Step 2: Motor Box/Mechanism/Conjoin
For the ride to work: First, turn on the motor and wait for the cart to reach the top. Turn off motor, then push the rod with the red gear away from you, and the cart will fall. Then pull back the rod to return the red gear to the blue gear.
Optional: You do not have to add the strings just yet.
Step 3: Tower/Track
(Color order doesn't matter)
Don't connect the tower to the base just yet!
Step 4: Cart
Add the cart to the track when you finish it!
Step 5: Track/Cart/Base/Conjoin
Step 6: String Attachment
You can use any string (I suggest soft twine, not dental floss).
In a worse case scenario, just turn the thing upside down and let the strings fall to the bottom, erm top.
Then fix the strings inside the grooves on the wheels, and you're mechanism is set!
Step 7: Extra Detail
Also: You can add vasaline to the tracks to help the cart run smoother.
And there you have it,
Another theme park ride!