Below is a passage from a book "RELATIVITY THE SPECIAL AND GENERAL THEORY by ALBERT EINSTEIN" and i am not able to understand the trajectories of the stone from the train and from the ground. I stand at the window of a railway carriage which is travelling uniformly, and drop a stone on the embankment, without throwing it. Then, disregarding the influence of the air resistance, I see the stone descend in a straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls to earth in a parabolic curve. I now ask: Do the “positions” traversed by the stone lie “in reality” on a straight line or on a parabola?"