You get a crazy idea to dig a tunnel through Earth from Boston to Paris, a surface distance of 5850 km, and run a passenger train between the two...

You get a crazy idea to dig a tunnel through Earth from Boston to Paris, a surface distance of 5850 km, and run a passenger train between the two cities in the figure. The train would move down the first part of the tunnel under the pull of gravity and then coast upward against gravity.

If you assume that the motion will be simple harmonic motion, how long will a one-way trip take?