Maybe my AMTRAK Bridge isn't your AMTRAK bridge ? There's the footbridge and then there's the Spuyten Duyvil Bridge which is for trains only.

For the footbridge, go across Dyckman Street until you're just short of the river. Take a right onto the service road that runs past the ball fields. I think there's a gate blocking unauthorized motorized vehicles from entering this road. On your right, just a bit before the end of the island, there's a staircase that leads to a bridge that crosses the tracks. That brings you to the western portion of Inwood Hill Then if you follow the second path on the right it will lead to the pedestrian path on the Henry Hudson Bridge.