The D&H bridge over I-787 carried what was once the Green Island Branch, which ran from Watervliet Jct (WX Tower/Cabin) to Green Island. The Troy and Schenectady Branch of the New York Central came through Cohoes at a higher level than the D&H, crossed the D&H Saratoga Division Main Line (Colonie Main) on a bridge, and joined the D&H G.I. Branch just east of 2nd Street (NY 32).
The D&H Troy Branch was the original main line of the Rensselaer and Saratoga Railroad. It left the Saratoga Division Main Line at JA (Waterford Jct) and ran south through Waterford and Green Island to meet the G.I. Branch, then continued into Troy on the old Green Island Bridge. It met the Troy Union Railroad at River Street, where one leg of the T.U.R.R. wye went south to Troy Union Station between Fulton Street and Broadway, and the north leg went toward the B&M at Hoosick Street.
As for history, the Green Island Branch is older than the hills. Same for the Schenectady and Troy Railroad, which was opened in 1842 and became one of the ten roads that constituted the original New York Central Railroad in 1853. The original R&S was opened to traffic from Troy to Waterford in 1835, with the cars being drawn across the Hudson River Bridge by horses account the bridge would not carry the locomotives. The Green Island Branch was in service by 1842, when the D&H granted trackage rights to the S&T over it to reach Troy over a better bridge.
The final Green Island Bridge was a vertical lift bridge, carrying the single main track of the Troy Branch, two lanes of highway, and a street car line. It was owned by the D&H, and was operated while the Troy Union Railroad was in service. I forget which year it was taken out of railroad service, but I rode trains across it in 1960, and the TURR was gone when I left Troy in 1963. The bridge remained as a highway bridge longer than that.