According to the Untermyer Gardens website:

The Gatehouse was built where the Greystone carriage trail crossed the Old Croton Aqueduct. The Croton Aqueduct, constructed to carry fresh water to New York City, ran 41 miles from Croton on Harmon into New York City and was in use in Untermyer’s time. Now the Old Croton Aqueduct Trail runs 26.2 miles from Croton-on-Harmon to the Bronx. (Note: I think there’s an error in this description. There is no such place as “Croton on Harmon”. The town is called “Croton on Hudson” and it’s Metro North railway station is called “Croton-Harmon”. According to Wikipedia: “During the days of the New York Central Railroad, the station and shops were known as Harmon. Trains continuing north of Harmon, including the flagship 20th Century Limited would exchange their electric locomotive for a steam or diesel locomotive to continue the journey to points north and west.

This is taken from the Vista Overlook. I’d heard that there were a couple of interesting reliefs on the gate posts and I wanted to go down to see them. However, I didn’t feel that I should leave the tour and I couldn’t go afterwards as we had to go to eat. So I went back yesterday. As the description above states the gatehouse is by the Old Croton Aqueduct trail so I thought I’d take the dog for a walk and also get to see the gatehouse up close. Unfortunately I started my walk down the trail too far away (virtually in Hastings-on-Hudson) and I never did get to the gatehouse (I had to get back to meet my wife). We had a nice walk though – on a piece of the Aqueduct trail that I’ve never been to before. So it wasn’t a complete waste. I intend to go back though.