The new viewing platform for the constellation public art project at Bannerman's Island, south of Beacon, NY proved to be a great place to watch trains. I was over by 6 this morning, to find a mist-covered river, but lots of train activity.
You can read about the project here.
Here's a southbound from the platform, a Northbound, and a close-up view of Bannerman's Castle.
The bridge, to the inbound side of the weekend-only Breakneck Ridge Metro-North stop, also provides good photo opportunities.