Make it till Mumbai... as crossing bor ghat is not to impossible for a bullet train to reach Pune...
We can have instead bullet train from Pune-Nagpur, but not Mumbai-Nagpur as it again crosses Thal ghat (1 option is there to run Mumbai-Nagpur bullet train is to make it run via Surat so that it can evade mighty Sahyadri range, as it can pass through Nandurbar region)