Types of Turkish Trains

Types of Turkish State Railways (TCDD) trains in Turkey. (For timetables, see the Turkish State Railways website).

Super Express Trains Süper Ekspresler
Fastest, most comfortable, most expensive long-distance trains, with few stops. These run from Ankara to Istanbul (Haydarpasa), Izmir (Basmane) and Erzurum, and vice-versa. Some are sleeping car trains.

Blue Trains Mavi Trenler
Fast inter-regional trains, with few stops, running on these routes (some are sleeping car trains):

Ankara - Adana

Ankara -Diyarbakir

Ankara - Izmir (Basmane)

Istanbul (Haydarpasa) - Adana

Expresses Ekspresler
Long-distance trains stopping at the larger cities and towns on these routes (some are sleeping car trains):

Istanbul (Haydarpasa) - Ankara

Istanbul (Haydarpasa) - Konya

Istanbul (Haydarpasa) - Denizli

Istanbul (Haydarpasa) - Gaziantep

Istanbul (Haydarpasa) - Kurtalan (east of Diyarbakir and Batman)

Istanbul (Haydarpasa) - Kars (note: 38-1/2 hours!)

Istanbul (Haydarpasa) - Tatvan (western end of Lake Van)

Ankara - Izmir (Basmane)

Ankara -Zonguldak

Adana -Elazig

Bandirma - Izmir (Basmane)

Eskisehir - Izmir (Basmane)

Isparta - Izmir (Basmane) (for getting to Lake Egirdir)

Kayseri - Adana

Regional expresses Bölgesel Ekspresler

Rail buses and Motor-trains Ray Otobüsü ve Mototrenler Regional short-run trains making all stops.

Normal Passenger Trains Normal Yolcu Trenleri
Cheap but very slow trains with both freight and passenger cars--avoid them unless you're trying to hide out from The Law for a long time!)  

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