Presuming that you will be riding the Doğu Ekspresi
(Eastern Express), you will have your meals in a dining car (Yemekli Vagon
Here is the Turkish State Railways restaurant car price list (in Turkish):http://www.tcdd.gov.tr/home/detail/?id=255
Breakfast (bread, butter, jam, cheese, egg, olives, tea) TL8
Coffee (Turkish, or Nescafe) TL2.50
Salads and Meze (appetizers) TL3.50—TL4.00
Grilled meats TL9.50—TL12.50
Beer or raki (one drink) TL6.50
It would be a good idea to take some of your own supplies, such as bottled water, dried fruits, biscuits, etc.
2000km on a train... Not for the faint of heart.