Serbia & Montenegro used to be one country following the post war break up of Yugoslavia. In 2006 Montenegro gained its independence and adopted the Euro as its currency, despite not belonging to the EU or being a Euro member.
Montenegro
I visited in 2007, taking a train from Bar on the Albanian border to Podgorica and then a bus to Kotor, before skirting the Adriatic Coast to Dubrovnik in neighbouring Croatia.
Kotor has a breathtaking situation on a deep blue fjord. The views from the castle ruins are majestic. The walled old town is beautiful too. In 2007 it was a bit of an unknown, but now it is hugely popular,
Serbia
Serbia was the biggest state in Yugoslavia and General Tito made Belgrade its capital after the Slavic nation was formed following WWI.
The city is an important rail hub and I have travelled there by overnight train twice, from Sarajevo and Skopje, and out again to Sofia & Romania.
Glad To See The Back Of Belgrade Dunav Station 
I was reading your book Travelling By Train Across The Balkans before the first lockdown hit. I was planning my maiden visit to the region and your stories were providing some great inspiration. Desperately hoping that I’ll be able to make the trip next year.
Glad you liked it. I really hope you make it to the region by train. Some real treats are in store. If you bought the book from Amazon I would be hugely grateful if you could leave a review. Thanks. Wilbur.