Serpentine.
Metro-North Railroad M7A no. 4198 (Bombardier, 1999-2006) is the lead unit on southbound Hudson Line train 758 seen slithering through the reverse curve north of Irvington Station.
With three lines radiating north out of Grand Central Terminal plus service to Port Jervis on the former Erie, Metro North covers a wide swatch of New York's northern suburbs with varying terrains.