The JŽ Series 611. Built from 1961-1963 at TŽV Gredelj. It was a diesel-electric unit, one of the first made from aluminium alloy in the world. There were 6 units made. The first examples of the Series 611 had a weak motor, but later models had a stronger one.
https://en.wikipedia.org/wiki/J%C5%BD_series_611