Shatabdi Exp - janam-janam ka saath hai, agle shatabdi tak - Ananya D'Souza

Not always 300 waiting List means there are total 300 people waiting for confirm seat.
Sometimes Berth multiplexing and RSA arrangement make Huge amount confirmations. Many a times Tatkal and Premium tatkal seats doesn't get filled up , they are also released for filling these Waiting Lists.
Thats why railways keeps a "Buffer limit" which is "Regret". If Regret status is shown then NO further booking is possible.
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in SL extra coaches are also attached to fill up the Waiting listers. So if they give just 60-80 Waiting Lists , then there is huge possibility that many seats will remain vacant after chart preparation.
And above all , its a way of some profits like for every last moment cancellation gives 80-100 rupees profit to Railways.