PLANNING - Initial Idea

My idea for my film is this:
-A group of girls have just finished a night out
-They part their ways one going one way, the rest going in the opposite direction
-The focus of the film is on the girl who goes her own way
-After they part their ways a dark figure appears out of the alley way behind them
-The girl notices him behind her but he then takes a turning, sge is safe; for now
-She continues walking until she gets to the tube station
-She gets on the tube
-She then see's the dark figure again in the opposite carriage walking fast towards her
-The train rocks, the lgiht flickers and he is right next to her
-The scene ends with her scream