In the moment only the night train is working. When the day train will be available is not known.

Yes, I just bought tickets yesterday

The tickets can be bought from the station at Topcider, Belgrade, where you catch the train. (Not from Belgrade Central Station.) I had to pay in Serbian Dinars. If you go on the night train expect to be woken up at the border crossings. I had a 2bed cabin but was on my own from Belgrade to Bar. The bunks are clean and reasonably comfortable. Take some food and drink - it takes 10 hours but can be longer and I didn't see the buffet car I caught the day train back - scenery is stunning and the 10 hours well worth the trip! If you are staying in Bar , a visit to the old city is well worth it. Taxi cost 3 Euros. I took the local bus to Tivat 7 Euros - took one and a h a lf hours with outstanding coastline - just stunning. And a visit to Kotor is easy using the bus. Well worth a visit. Hope This is useful.

How and where do we get tickets for the Belgrad to Bar train?

Consider going all the way along the coast to Ulcinj. Its 45 mins from Bar. Ulcinj has a completely different feel to anywhere in Montenegro because its population is actually 65% Albanian. Very interesting place. Strong muslim feel. One week in Podgorica is a long time. Bay of Kotor is really nice. I would stay outside of Kotor in Muo, Prcanj, Stoliv or Perast, & do day trips in. Cetinjie is worth a look for a day. Of course a lot depends on your likes. Inland great for hiking, walks & historical rural feel. Coast very nice if you like the water, restaurants, beach & sunshine. Enjoy. You'll love it!

Yes