Vangölü Ekspresi (‘Lake Van Express’) departs Istanbul’s Asian station (Haydarpasa) twice a week and arrives in Tatvan station after a 42-hr journey. This train also calls at a number of cities and towns across Asian Turkey, including Eskişehir, Ankara, Kayseri, Sivas, Malatya, and Elazig among others. Inter Rail pass is accepted in this train. See Turkish railway authority’s website.
Train ferries connect Tatvan with Van, a quite big city located on the eastern shore of Lake Van. These ferries accept pessengers no matter whether they have a connecting train ticket or not.
The telephone code of Tatvan is 434 (+90 434 when calling from out of Turkey).
Erzurum to the north; Malatya to the west; Diyarbakir, Hasankeyf, and South-eastern Anatolia to the southwest; Van to the east, across the lake. If you have your passport ready with a visa, why not try moving on further east to Iran?