Joshi, who was scheduled to lead a delegation to Finland to sign an MoU in road transport sector, cancelled the foreign visit in order to assume additional charge on Monday.
Joshi had held the Railway portfolio for a brief period in September last year before Bansal was given charge of the ministry.
In his resignation letter, Bansal maintained he was not aware that his nephew was in contact with Railway Board Member Mahesh Kumar but he still thought it proper to step down.
CBI had earlier grilled Bansal's personal secretary Rahul Bhandari, a 1997 batch Punjab cadre IAS officer who is believed to be privy to all the crucial decisions taken in the Railway Ministry.