The correct answer is 30 minutes.
During TFC on single line, when line clear is obtained for more than one train, trains are run at an interval of 30 minutes. This is to ensure that there is enough time for the trains to pass each other safely.
Option A: 20 minutes is too short an interval. This would not allow enough time for the
132.3s0 89.4 11.4 132.3c6.3 23.7 24.8 41.5 48.3 47.8C117.2 448 288 448 288 448s170.8 0 213.4-11.5c23.5-6.3 42-24.2 48.3-47.8 11.4-42.9 11.4-132.3 11.4-132.3s0-89.4-11.4-132.3zm-317.5 213.5V175.2l142.7 81.2-142.7 81.2z"/> Subscribe on YouTube