Break the trip down into smaller chunks! You won't get sensible results trying to generate an itinerary from Netherlands to Turkey in one search.

Most journeys are going to involve getting first to Budapest, then on to Bucharest either with a very long day train or by the well-timed sleeper. Of course you can stop off somewhere en-route if you prefer.

Then (in summer) a direct sleeper from Bucharest to Istanbul.

You can use this London-Istanbul page as a guide. Only the dark blue route is possible right now. From Leeuwaren, you'd join that route in Germany or Austria I guess.

https://www.seat61.com/Turkey.htm

If you want to use another sleeper, there's one from Amsterdam to Vienna.

And you will have better luck using www.bahn.com to search timetables. It's more up to date than Eurail.

But always try to break it down into smaller sections.

Thank you Ralderton.. i will check it out.. thanks for this usefull info..

I will post the outcome,,

I,am in Istanbul now.. the only bad luck was in Germany.. due to wheater conditions, all public traffic stopped. around Munich.. Insbruck.

So i lost all my planned connections, and it was 1 day more to travel.

In Rumania i bought a ticket by myself, and because of the time slot before the new next day, it saves me a travel day on my pass..I still have 7 left.