As far as I know, the 1st digit signifies the Zone to which the starting station belongs, and the subsequent 2 digits represent the PRS counter associated to the ticket.
Not all Rajdhani/Shatabdi PNR start from 2. You can check on the PNR form and you will find several PNRs for DBRT Raj and HWH raj starting with 6. Usually for SCR and SWR trains, the PNR starts with 4 and for NR/NCR it starts with 2. I am not sure about the exact number for each zone though.
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more... The remaining 7 numbers are randomly generated to give a unique PNR.