The Gallilean train moving at constant velocity is used as an example of the inability to detect contast motion; however, one can go much further than this and say that we can only detect relative motion. Only relative motion has any reality. Imagine that every particle on the train is acted upon equally by some external force at all times as opposed to the wheels being acted upon by a force, which is then trasmitted to the body of the train and subsequently to the passengers. One can see that even external forces of this type are utterly undetectable. No force would be felt! An accelerometer can detect accelerations only when the motion of the reference mass differ from those of the constraining walls. An accelerometer cannot detect gravity (neglecting tidal effects)! Everything will be in free fall unless acted upon by an external force.
Similarly, no matter what the motion or derivative of that motion, it is utterly undetectable as long as all the particles experience it equally. When an accelerometer on the Earth's surface measures a = g, what we actually detecting is an acceleration upward which is transmitted from the electrostatic forces acting on the body of the accelerometer and, in turn, being transmitted to the reference mass.
Notice that the direction of the acceleration on objects on the Earth's surface is upward, not downward!
Any accelerometer will accurately reproduce the fact that we are accelerating upward on the surface of the Earth. If one tries to tare out this natural value so as to agree with individuals' preconception that we are not accelerating on the surface of the Earth, then flipping the accelerometer over will result in a different value (a = 2 g). This is obviously nonsense because the correct value must be independent of orientation of the accerlometer. Only one value will yield orientation independence and that is a = g (directed upward). This gives us a special reference frame, called the inertial or freely-falling frame in which the accelerometer reading is zero.
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Mass pulls on space and space on mass. All masses only react to the local curvature of space (the energy field). This explains action as a distance forces such as gravity. Similarly, charge acts on the electric field that permeates all of space and distorts it and other charges interact only locally with that field.
