A ball is released from the window of a train moving along a horizontal straight track with constant velocity. The path are observed:

For the ground observer,

                  $x=ut$ and $y=\frac{1}{2}g{t}^2$,

         $\therefore y=\frac{1}{2}g\frac{x^2}{u^2}$, it represents a parabola.

For observer inside train, 

               $x=0,$ and so ball appears to fall vertically.