Hellenic State Railways or SEK (Greek: Σιδηρόδρομοι Ελληνικού Κράτους, Sidirodromi Ellinikou Kratous; Σ.Ε.Κ.) was a Greek public sector entity (legal person of public law, Greek: Ν.Π.Δ.Δ.) which was established in 1920 and operated most Greek railway lines until 1970.