Istanbul to Sofia Train

I'm surprised there's even a train option at all, but it seems there may be. I haven't verified this by going to the appropriate rail-company websites, but it looks as if, in addition to all the stops during the night, there is also a 3-hour layover in Kapikule between around midnight and 3 AM--I guess at the border. That should be fun (not).

Also a concern is that both the Halkali-Kapikule and the Kapikule-Plovdiv segments run only once a day. One cancellation and you are well and truly stuck. I traveled through the Balkans in 2015 by train where possible and by bus the rest of the time. I met a tour group from Canada along the way. They were on a train-based tour of the area. The group had already had two night trains (with some sort of sleeping accommodations) canceled on them, leading to two overnight bus rides. They were taking it far better than I would have--not that it was the tour company's fault.

Rome2Rio.com, which is most definitely not always reliable in these matters, thinks there are two companies providing bus service. Unfortunately, both seem to be overnight buses. Those have one advantage: relatively light traffic, meaning faster travel time. But they have two significant disadvantages: the really miserable prospect of sitting up all night long and the safety issues inherent in traveling on long road trips at night, especially in that part of Europe. Have you noticed how often horrendous bus accident happen at night? Bulgaria has Europe's second-highest rate of road deaths per capita.

I'd explore flight options on skyscanner.com. Turkish Airlines does fly that route. It gets good reports from folks who have taken its trans-Atlantic flights.

Seat61 is definitive for all things rail in Europe
https://www.seat61.com/international-trains/trains-from-Istanbul.htm

Looks like its still suspended because of Covid - hopefully back by summer. Yes you have to get off at the border - that's not uncommon. Its totally on my bucket list - and the train looks cheap and comfortable.

Looks like you can get to the border either during the day or evening spend the night and travel on the next day/night either by train or bus https://www.seat61.com/train-travel-in-turkey.htm#Istanbul_to_Edirne

This will be much easier to figure out once you're in Istanbul - you can't book ahead anyways

Thank you so much for the feedback. I’m sitting here reading about the Russian invasion to Ukraine and possibly seeing my travel plans spoiled by greedy politicians!

Both Turkey and Bulgaria are members of NATO - I wouldn't cancel your travel plans yet. Putin won't take on NATO because he doesn't want to lose