The actual path from the HH Bridge down to the Amtrak Bridge is nearly impossible to see on a map because it's a short unmaintained path covered with trees, but you can see the Amtrak Bridge, that crosses the tracks on this map.
http://maps.google.com/maps?q=inwood...+york&t=h&z=18
Basically you exit the bridge on the west side and go south, down towards the river. The bridge is a short ways, on your right, and you just go down the steps, then follow the path that runs south along the river to Dyckman.
Then go left on Dyckman for about a block, and then right, up a hill on Staff Street. At the top of the hill, Riverside Drive, you'll see an underpass on your right. Just past that underpass there's a staircase that leads up to the Greenway. From there the Greenway runs all the way down to the Battery.