Actually,
Delhi-Mumbai : 2 Rajdhanis (each daily), 1 Duronto (biweekly), 2 Garib Raths (3+4 days=daily)
Delhi-Kolkata : 2 Rajdhanis (each daily), 2 Durontos (4 days a week and 2 days a week=6 days a week)
Delhi-Bangalore : 1 Rajdhani (daily), 1 Duronto (weekly)
Delhi-Hyderabad...
more... : 2 Rajdhanis (daily and weekly), 1 Duronto (biweekly)
But
Delhi-Chennai : 1 Rajdhani (biweekly), 1 Garib Rath (weekly), 1 Duronto (biweekly) :P
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Chennai Rajdhani's frequency should be increased to daily :)