Complex sentence: The main source of the ore were mines near Kryvyi Rih (then in the USSR, now in Ukraine), from where it was transported by rail via Medyka, Przemyśl, Tarnów to Jaworzno Szczakowa.
Split into simpler sentences: The main source of the ore were mines in the USSR (now in Ukraine), from where ore was brought in by rail. The ore was moved from the mines in Kryvyi Rih via Medyka, Przemyśl, Tarnów to Jaworzno Szczakowa.