Balasore accident happened due to signal failure? What was the main reason for the accident?

In Odisha's Balasore accident, Chief Secretary Pradeep Jena has made a big claim that 275 people died in the accident. While Rahul Gandhi is targeting the Center regarding this, Railway Minister Ashwini Vaishnav has demanded a CBI inquiry. So the question arises that what was the real reason behind the accident.