What do you mean by every possible stop ? I mean you could spend your whole life going to Istanbul !

I wouldn't do this route in less than a week but it could be done in more or less days.

Count how many pass days you might need. If you spend a full day in Vienna there's no need to use one.

As for route ideas (there are many) :

- London - Paris or Brussels by Eurostar (passholder quota so book well in advance)

- Paris/Brussels - Vienna night train

- Vienna - Budapest daytime train

- Budapest - Bucharest night train

- Bucharest - Istanbul : direct night train in summer (24h journey), otherwise change somewhere in Bulgaria the rest of the year

Please note that there are no international trains into Belgrade and Serbia. The easiest route is thus via Budapest and Romania.

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

There’s an excellent write up of the journey from London, via Budapest and Bucharest, here. (The alternative route via Belgrade is much harder at the moment, due to a lack of trains.)

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

It takes about 4 days of travelling, but I would suggest you spend at least a week or more on the journey if you want to see some sights along the way.

I would suggest a good read of that page, and come up with some places that appeal as a stopover. 

If you scroll down to the “How much does it cost” section on that page, you will see that he recommends a ‘10 day in two months’ pass for a return journey. Total cost for the pass + return reservations is in the region of £540 for an adult. If you wanted to spend a bit longer than 5 days each way, you could upgrade the pass for example an extra 5 days would add about £65 to the cost.

There are a lot of options, and part of the fun is looking at timetables and maps, and finding out where you’d like to stopover. It can be a bit of a logistical challenge, but it’s easier if you breaks down like this.

1. Get to Budapest. You can go via Paris and Switzerland, or via Brussels and Germany into Austria, or even via the Czech Republic. A million places worth stopping off! I can recommend bars in Berlin, a gorgeous Alpine hut in the Austrian Tyrol, a spa in Brno. You could do this in 24 hours, or spend a month.
2. Budapest - Bucharest. About 15 hours. There is one (long!) day train, but the sleepers (there are two per day I think) make a lot of sense here. Of course yo might stop off in Transylvania, rather than going non-stop to Bucharest.
3. Bucharest - Istanbul. Direct sleeper in summer. The rest of the year, you’d have to change a few times then board the sleeper at Dimitrovgrad. This is another long journey, so well worth stopping off somewhere. Maybe Veliko Tarnovo, as suggested.

Use the Seat61 page as a guide, and then mess around with timetables on www.bahn.com. (Search for dates next week, rather than far in the future when timetables aren’t yet published.) It’s really down to you to decide how long you want to spend, and where appeals the most.

3 even. Still many people say "the night train” as if there's only one...