İstanbul has a large railway network comprising both Asian (between Sirkeci and Halkalı) and European (between Haydarpaşa and Pendik) sides. To access the trains you can use either İstanbulCard, Akbil (the smart ticket) or tokens. There are also fast trains and normal trains leaving from Söğütlüçeşme station to other major cities in Turkey. To reach other cities most of the lines make a change in Ankara. The major cities rachable by railways include Adana, Afyon, Ankara, Balıkesir, Bandırma, Diyarbakır, Elazığ, Eskişehir, İzmir, Kars, Kayseri, Konya, Malatya and Van.
For more information: http://www.tcdd.gov.tr/