Sentence:  The `` moving '' picture of the train or the <WSD>wave</WSD> coming at the audience is , to be sure , more intense than a still picture of the same subject , but the difference is really one of degree ; the cinematic element of time is merely used to increase the realism of an object which would still be reasonably realistic in a still photo . 
Meaning of the word marked with <WSD> tags:  one of a series of ridges that moves across the surface of a liquid (especially across a large body of water)