There is usually a space under the platform for someone to hide under, but this is not always the case and depends on the station. A notable one would be 14th st union sq where the platform is mechanized and slides out at certain points on the curved station track and therefore can't accommodate a space.

Other stations have slots built into the walls to fit a grown man easily if you stand up and lean into them. I have seen workers use them in this capacity.

There is however a channel cut into the entire length of the track of every station large enough for a man to lie down completely and not be hit if you go into a prone superman state and let the train pass over.

The easiest would be just to walk over to the otherside if it was a double sided track station. There are stairs at each end of the platforms to walk up behind the swinging barriers.

I've been down on NYC tracks before and it is a little bit of a jump and a push to get back up. For comparison the T in Boston is barely a step high in many stations.