You can buy high speed train and main line train tickets via TCDD Tasimacilik’s online ticket system:

Buy online train ticket 

Note that;

• TCDD Tasimacilik directly sells tickets. Rail Turkey has no interest nor responsibility.
• You can reserve, buy, change or refund your tickets via this system.
• You’ll need PNR number to list your reservations/tickets
• HST tickets are on sale prior to 15 days to departure. This is 30 days for main line train tickets.
• When choosing train station, you need to use Turkish characters when needed. Note that “I” and “i” will display different results. Use “i” for Istanbul.
• You cannot buy international train tickets online.
• Gender decleration is important. Wrong declaration may cause trouble.
• SMS sent by system or print of online ticket will be enough. For HSTs, valid ID matching the info on ticket will even be enough.