2-week train itinerary from Istanbul to Berne in April

Great idea!

There is a single train from Turkey to the rest of Europe : the daily Istanbul - Sofia night train (in summer there is an extra couchette carriage towards Bucharest). You can’t book this train online : it’s a large train though so availability is rarely an issue outside summer. Definitely go to the station as soon as you arrive in the city. More details : https://www.seat61.com/trains-and-routes/bucharest-and-sofia-to-istanbul-by-train.htm → note that there are passport controls at 2am.

Afterwards I’d head to Craiova or Bucharest, either way it’s a full travel day (daytime trains). Transylvania is a beautiful region so I’d spend a night or two in Brasov or Cluj.

Then take advantage of the multiple night trains towards the west : it’s a rather dull and flat area anyway. Only the one to Vienna (stopping in Budapest at 5am) is available online, others must be booked at a Romanian counter.

After Budapest, trains are plenty and it’s easy to move around. There are night trains to Bregenz/Zurich but actually I’d recommend doing the route with daylight → Alpine scenery! Stop in Vienna, Salzburg and/or Innsbruck.

Helpful reservations guide : https://www.seat61.com/interrail-and-eurail-reservations.htm