As we were leaving I had a good look at the gate. The sign says:
THE BRIDGE ENTRANCE GATE AUTOMATICALLY CLOSES AT DUSK
PLEASE LEAVE THE PARK WHEN THE SIREN SOUNDS AT 30 MINUTES PRIOR TO PARK CLOSURE
I was intrigued by the technical and commercial aspects of such a system, however it was clear that by 'automatically' they meant that someone would lock it without checking if anyone was still there.
Afterwards we returned to the Waterford Locks on the Erie Canal. Last time they were drained and deserted and the canal was frozen over. This time we watched the boats (and water) coming and going in the sunshine:
After that we had a very nice late lunch at the Albany Pump Station followed by watching Andy Murray's semifinal in the US Open down in NYC. Very satisfactory.