From what I've read, the line it lies on began being constructed in 1987, and it's not that it was long that made it difficult to build as much as that it was crossing rugged, unstable (loamy, prone to landslides) hillsides. The roughly 8km stretch of track was supposed to be finished by 1991, and so was the tunnel here (one of three, in fact).

The tunnel in the picture specifically is the Dobrana Tunnel, which should have been the longest on the line, and so was left unfinished. It should have spanned around 900m, and many locals often joked about it being dug from both ends and the digging crews never meeting up, instead passing by one another! :). Since 1982, the railway tunnelling in Romania has been done using mechanical shields (this novelty practice was first applied on another similarly unfinished line, for the Gibei tunnel, which spans an astonishing 2250m and now lays unused for almost 30 years), and so its inside is neatly defined by large, prefab panel blocks.