What is this? Federal owned ROW in metrowest Mass

Different federally owned ROW: Anyone know why the end of the cape cod line is owned by the feds in Wellfleet? I would guess there was some sort of naval port out there at one time? Anyone know exactly?

During WWII I believe the Sudbury ammo depot worked in concert with the South Weymouth Naval Depot, which was where the ammo was transferred from rail to water. I expect the rail movements went south and around on the former Old Colony line through Framingham and Mansfield rather than over the Grand Junction, but I've only seen a few mentions of this in RR history publications.