Complex sentence: Between 30,000 and 45,000 people are believed to have attended this concert, a few thousand of which were stranded at Narara railway station, believing the concert was being held in Narara.
Split into simpler sentences: Between 30,000 and 45,000 people are believed to have attended this concert. A significant portion of these people were stranded at Narara railway station, believing the concert was being held in Narara.