This massive storage yard (which we passed once before at street level) serves the nearby Linden Shop, where the MTA fabricates all of the track panels (pre-assembled sections of subway track) used in rehabilitation projects on the aboveground subway lines. After being delivered from the shop to a work site, the panels are lifted by crane onto the tracks, where they replace the old rails and ties. (Due to space constraints in the tunnels, underground tracks have to be constructed on site.)
The track you see curving overhead runs from the New Lots Line down to the Linden Shop, about a mile from here, merging with a track that splits off from the nearby Canarsie Line along the way. The combined track connects to the LIRR's Bay Ridge Branch; this is one of only two connections between the subway system and the national rail network, and it makes this yard a convenient location for storing materials and equipment delivered by rail from elsewhere in the country.