Hey, thank you for your answer. I am only in Istanbul for less then 2 days, I am staying at the european side of the city vey close to Hagia Sophia. Just what you can not miss when being in Istanbul I mean food, stuff you can get there, where to go, what to see. Two days is not enough but I will see how it goes, might stay a bit longer. Train to thessaloniki is also important. I am sure Hostel staff will help me with it once I am there but I just like to know a bit before my journey :)
Thank you so much
Thomas