Section 19(1)(iv) in The Delhi Metro Airport Express Line, General Rules, 2010
(iv)normally signaling system will permit only one train at a time in one Tunnel Ventilation Section, more than one train in one Tunnel Ventilation section can be sent only in case of emergencies, and in a case the Traffic Controller decides to send additional trains in a Tunnel Ventilation section already occupied by a train, he shall set the route manually for additional trains and shall advise train operators about this before authorising them to proceed;