due to air density and g force on the surface.
1. What is the density of air?- preferably in metric units.
2. What do you mean by g force on the surface? How is g force on the surface any different from g force a little underground? The symbol "g" is commonly used to denote acceleration due to gravity. Do you know and understand anything about Newton's Law of Gravitation?
1 The density of air (at sea level) is 1.225 kg/m3.
2 I only know what Wikipedia says about g force plus having a ride on a rollercoaster which would be a bit like doing 1,000kph on the surface or even a little under the surface.
Any train doing 1,000kph will need a sometimes very deep straight tunnel.