Turkish Train Monthly Rail Passes

Interrail tickets and Eurodomino rail passes sold by Rail Europe are valid in Turkey, but the TCDD Train Tour Card (see below) is probably a lot cheaper.

The Balkan Flexipass available from Rail Europe (good for Turkey, Greece, Bulgaria, Romania, Macedonia and Yugoslavia [Serbia & Montenegro]) is also valid in Turkey.

TRAIN TOUR CARDS
Turkish State Railways (TCDD) offers Train Tour Cards (Tren Tur Kartlari) good for 30 days' unlimited travel on the entire Turkish rail network.

—Express Train Tour Card (Ekspres Tren Tur Karti) costs TL70 (about UK£30/US$52/€43)

—Sleeping Car Train Tour Card (Yatakli Tren Tur Karti) costs TL300 (about UK£128/US$223/€185) and is valid both for carriage on any train, and use of any sort of sleeping accommodation: sleeping car (yatakli vagon), couchette (kusetli), or couchette with sheets, blankets and pillow (örtülü kuset).

Note that you should reserve any sleeping accommodation in advance.

If you take more than 5 one-way (single) journeys, or 3 round-trip (return) train journeys, you will save money with a Train Tour Card. (See also Train Travel Costs.)

However, the trains don't go everywhere in Turkey; except for the Istanbul-Ankara route, they're often slower than the bus; and outside of the Istanbul-Ankara route, the equipment may be well-used, and in some cases even dilapidated—though serviceable.

Plan your itinerary, and see if a Train Tour Card suits you.
 

PURCHASING TRAIN TOUR CARDS
Have your passport or official photo identity card ready to show when you buy and use your card. The card's 30-day validity begins on the first day you use the card (which may or may not be the day you buy the card).

You can buy cards at major stations, including these:

Source : http://www.turkeytravelplanner.com/trans/Train/monthly_passes.html