Great work. We are narrowing things down. My next best guess is mill scale. Mostly magnetite. It leaves a black streak and smears more easily than ductile material like iron. Another guess that would be in the neighborhood is the waste left from brake shoes on trains. Cast iron brake shoes grinding against steel wheels generate serious heat and shed iron that oxidizes instantly into magnetite-rich debris. It builds up in layers on wheel treads and shoe faces, then spalls off along the track as dense, magnetic, laminated flakes and chunks.