Srbija Kargo 661 256Kalemegdan Kalemegdan

A train in the Park. Is this a Main line? was there another track on the left side?

This is not main line, there a line Pancevacki most - Vukov spomenik - Rakovica which 90% of the route is in tunnel below Belgrade. But, dangerous materials can not go in tunnels beneath city, goes here on bypass tracks. This locomotive was sent to the station Beograd Spoljna, to his train to Vrsac.
Yeah, you noticed very good it was once here a second track at left.