Which journey are you looking to make ?

Trains are sparse in the Balkans and international trains are non-existant with an exception or two. The Eurail map you might've seen is very outdated.

These railways don't share their data so that's why you can't find the timetable, anywhere but on their own websites.

Reservation is not possible online, you'll have to book locally at a ticket office.

thank you for your reply. I am planning to take a night train from Beograd to Podgorica. It says that reservations are required, but since it doesn't appear on the list, I can't make reservations online. On the day of my ride, I make a reservation at the station counter. Should I show my pass at that time?

Seat61 is usually the best place for this sort of information. You can only buy the Eurail reservation at the station. If you are in Belgrade for a few days, try to buy it as soon as you arrive, otherwise come to the station in good time on the day of departure.

https://www.seat61.com/interrail-and-eurail-reservations.htm#Serbia

Just show them your rail pass if they ask for it.

This page on Seat61 suggests the train leaves Belgrade at 2030 every day.

https://www.seat61.com/belgrade-to-bar-railway.htm

Thank you everyone for the information. I'm planning to ride from Beograd to Podgorica next month and will be visiting Bucharest, Prague, Warsaw, etc. It's my first time using EurailPass, so I'm a little nervous, but I'm really enjoying it. Seat61's website seems to have a lot of useful information.