  The time it takes to fall down a tunnel through the center of the Earth to
the other side takes approximately 42 minutes, but only when given several
simplifying assumptions: a uniform density Earth; a gravitational field that
varies linearly with radial position; a non-rotating Earth; a tunnel evacuated
of air; and zero friction along the sides of the tunnel. Though several papers
have singularly relaxed the first three assumptions, in this paper we relax the
final two assumptions and analyze the motion of a body experiencing these types
of drag forces in the tunnel. Under such drag forces, we calculate the motion
of a transport vehicle through a tunnel of the Earth under uniform density,
under constant gravitational acceleration, and finally under the more realistic
Preliminary Reference Earth Model (PREM) density data. We find the density
profile corresponding to a constant gravitational acceleration better models
the motion through the tunnel compared to the PREM density profile, and the
uniform density model fares worse.
