How to get from Dubrovnik to Athens by train

Not.

Dub has not had trains since the tiny ones on the narrow gauge were withdrawn long before 1970.

Since covid and some other political problems in this area about all trains over the border have been cancelled and local trains IN those small countries are very sparse and often unreliable.

The usual way to reach GR is via ferry from ITaly-and Dub or nearby (at least summertime) also has ferries to IT.

For overland route with best use of trains on rails-thats for dedicated railfans who go out of their way to sit in these iron monsters. Check seat61com for best overview.

Best line to use in that case: Bar (end of line, Monte Negro, reach by bus via coastal road)-to Beograd-should have some great views for the daytime train and there is an overnite. And then you get stuck in white city (=B-gr).