Einstein:Book chapter 06 - The Theorem of the Addition of Velocities employed in Classical Mechanics

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LET us suppose our old friend the railway carriage to be travelling along the rails with a constant velocity $v$, and that a man traverses the length of the carriage in the direction of travel with a velocity $w$.
How quickly or, in other words, with what velocity $W$ does the man advance relative to the embankment during the process? The only possible answer seems to result from the following consideration: If the man were to stand still for a second, he would advance relative to the embankment through a distance $v$ equal numerically to the velocity of the carriage. As a consequence of his walking, however, he traverses an additional distance $w$ relative to the carriage, and hence also relative to the embankment, in this second, the distance $w$ being numerically equal to the velocity with which he is walking. Thus in total he covers the distance $W = v + w$ relative to the embankment in the second considered.