Going from memory
If you do straight lines from any point on the surface to any other point -- assuming equal distance from the global center -- and drop something in with no friction, the actual transit time will be the same for all tunnels. Shorter tunnels will have slower accelleration/decelleration. Longer ones (closer to the core) will have more extreme ones. But the actual transit time for all of them comes out the same.
In short, you'd need to add enough energy to the system to overcome the friction, and to make any trip between near neighbors take less time than it already does. (And no one wants to hear about any new transporation that takes longer than current methods.) Any trips that are long enough that gravity train trip times were comparable to air travel would be down past the crust.
