I am not sure if DOS was introduced later or simply from Day 1 of this train's launch. IR may have thought of giving DOS to the Ranchi Rajdhani via Gaya since people and politicians were demanding it as a stoppage for ER Rajdhanis. Dehri-On-Sone serves as the entry corridor for the Rohtas District Head Quarters near Sasaram + It has a Cement Plant. It is a booming township in the district so trains like the Poorva Exp via Gaya, Kalka Mail, Kolkata-Jammu Express, Doon Exp, Jodhpur Express, Lal Kuan Express etc are not enough to fill it's appetite... now latest demands are Durgiana Exp and many Rajdhani Trains ;-D