A lot of these trains are suspended because of covid or because of covid, sadly. A few suggestions:

• There are no trains between Timisoara and Belgrade. Travel via Zagreb to Belgrade, or travel via Bucharest to Sofia instead. 
• The only international trains to/from Bulgaria are to/from Romania, as of a week ago. Situation might have changed in the meantime. I would suggest to take the route Budapest - Timisoara - Bucharest - Sofia. There should be a direct train Bucharest - Sofia (via Ruse), otherwise you can also travel via Craiova andVidin. 
• Same goes for Thessaloniki - Skopje. An alternative would be train Thessaloniki - Florina, taxi to Bitola, then train Bitola to Skopje

I did exactly that in 2007. The taxi from Bitola to Florina was 30 EUR (and they accepted euros). Taxi drivers were awaiting the arrival of the train in Bitola. Agree on a price before you get in a taxi.

Additionally, there are no international trains from/to Albania. Domestic trains are sometimes running and sometimes not...

@Henri Pohl  you asked me in Facebook already on top of my answers i recommend to get in contact with @runner.on.rails  he have actual the same problems in Skopje :)

Hello all,

Same here these days. Stuck in Sofia. Wanted to go to Thessaloniki or Tirana. Both options no (Eurail) trains running. Looking for busses now but no results yet.. :)

Greetings,

Arthur.

IF in Sof-look for buses with your own eyes on offices, not on this www. The main station in Sof for the trains has been renovated , as far as I´ve seen since my last visit) and to the left when exiting it is the BAS-=Autobusni Kolodvor-with many desks. There used to be also a hodge-podge of more basic huts/tiny offices in front of station-but I think this mess has been cleaned up now. Most buses they advertise come from TR and their timings can be quite unpredicatble. many are not on www at all.

Also most hoStels will know or have even contacts as so many people ask for that. Best was hoStelMoStel, in a kind of inner courtyard near centre.

If you are married with trains and/or determined to go over rusty rails on vandalised trains as mcuh as can: IN BG/BDZ as far as border=KULATA, then there is a gap of some 30/40 kms till first stop in GR where OSE, now Hellenic trains, runs a very few to Thess-and more to Athinai (I always forget this name)-check seat61.com, it may have advice.

seat61 also now has detailed advice+ rec bus to bridge the gap into Serbije.

Direct to Skopje BUS is and remains always best-EU is somehow trying to get the locals together to fund some railway-there has never been any till now.