A few months ago I was surfing idly and found this little thing called a Gravity Train. For those who do not know what it is, see http://en.wikipedia.org/wiki/Gravity_railroad" [Broken]. After reading about it I thought maybe I should try and find out how much time it would take (42.2 minutes) but anything I tried did not work. Finally, when I decided to give up and sneak a peek I saw that they calculated it considering it as an oscillation (which in a way it is), but what I have been trying to do is calculate that time using kinematics. I know the relation of 'a' with displacement (x), a = g((R-x)/2R) where g = 9.8, r = radius of earth, x = the depth to which the body is inside the earth. Whatever approach I take from here leads me to a dead end. Any ideas how I should proceed?