Hellenic State Railways or SEK (Greek: Σιδηρόδρομοι Ελληνικού Κράτους, Sidirodromi Ellinikou Kratous; Σ.Ε.Κ.)

During this period very little expansion of the existing network took place, the most notable being: A contract was signed in 1928 for a new line connecting Kalampaka to Kozani, but the project was abandoned in 1932 due to lack of funds.

The Greek railway system (both infrastructure and rolling stock) suffered serious damages during World War II (especially in 1943–1944) and did not become fully operational until 1950.

They also continued to use an assortment of older types of locomotives inherited from the previous Greek and Ottoman railway companies.

Additional metre gauge multiple units were inherited from networks absorbed in SEK.