Mark Smith, train expert at Seat61, says the 296-mile journey between the Serbian capital and Montenegro's Adriatic coast is one of Europe's most spectacular train rides.
"I think it's absolutely epic. It summits at over 3,000 feet above sea level in the breathtaking Montenegrin mountains," he says.
"It's also an engineering marvel with 254 tunnels and 435 bridges, including the world's highest railway viaduct," adds Smith.
And it all costs just $22 a ticket.