The third rail is on the outside (could be either side though) of the main rails. It typically has a wood or other insulator covering, is mounted on insulated stand-offs and is energized at a nominal 600 volt d.c. in reference to the main rails. The pick-up mechanism on the train, a sort of paddle like device contacts this rail to pick up the power for the traction motors. There is minimal clearance between the main rail and the third rail, and minimal clearance under the pick-up device. If you were to fall and land between the main and third rail it would be a miracle if you did not get crispy fried..
In many stations and for portions of the right of way there is a gap between the ties of the main rails that provides a depression that in a best case scenario would allow for a person caught on the tracks for whatever reason to lie down in this depression with, again in best case scenario, sufficient room for the trainset to pass overhead without contacting the body.
I do not recommend testing the clearance using your own body.
I’d always assumed there was room underneath a train, either a subway train or a regular surface train, for a person lying flat to have the train pass over him harmlessly. Are you saying that the bottom of a train is so close to the ground that there isn’t even 18 inches of clearance between the ties and the underside of the train?
It is a bad day when I don't learn something new.
Thank you once again.