You can catch from Bucharest a direct train to Istanbul
My suggestion is to take the early Nighttrain from Budapest starting arround 3pm (15.00) from Budapest Keleti to Bucharest and connect there for the seasonal direct carriage Bucharest - Istanbul.
Please note the average speed in Romania & Bulgaria is pretty slow and the trains have at the borders passport checks as Romania and Bulgaria are not in the schengen zone this means it´s take quite long for this journey.

Go to THIS site and see all there is to know: seat61.com. DO read till very end, as there are many updates, and the route via Beograd/Old Yogoslavija is NOT feasible anymore=this means there is only the very same way to come back by train as going out.

And prepare yourself for delays, and also make up mind to tell more exact where to start from and what the problems in fact are.

Hi Thanks for all the replies.

I am starting from london to paris-munich (overnight in Munich) then Budapest-Bucharest.

Problem starts in bucharest as train journey to Istanbul shows 42 hours and 2 train changes. Guess we get off the train in Gorna wait for the next train for 14 hours then another train to plovdiv get off wait for 9 hours and then take istanbul train.

If l get the train from Bucharest to Sofia then Istanbul if anyone have any experience would be much appreciated.

Also l am making this journey with my dad and m 7 year old daughter just to explore couple of cities on the way l dont know how wise l am being :)

Thanks

No no Bucharest - Istanbul takes long but not 42hrs

The Railplanner doesnt have all trains In Summer there is a direct carriage that does the journey in sheduled 20hrs (please note you will be woken up at the Bulgarian / Turkish border and have to disembark with all your luggage for Immigration and Customs control and then reboard the train in the middle of the Night) https://p.bdz.bg/www/s/o/sofia-istanbul-sofia-2-2911.pdf