It had earlier been estimated that the six train sets would come at a cost of Rs. 25,000 crore.
Bullet trains - designed to run at a speed of 325 km per hour (kmph) - are run on the standard gauge; but the railways wants to run these at a speed of 110 kmph on BG routes.
The railways will also need to carry out massive upgrades in the track structure and signaling systems - which will also come at a huge cost, officials said.
The railways plan to build a "Golden Rail Corridor" for running these slow-moving bullet trains on four other routes apart from the Delhi-Mumbai sector. Routes identified for this corridor includes the Mumbai-Kolkata and the Chennai-Bangalore sectors, besides the Delhi-Jaipur and the Ahmedabad-Mumbai routes.
"Most advanced countries have switched over its locomotive hauled conventional inter-city trains to Electric Multiple Units (EMUs) train sets for operating speeds ranging from 130-160 kmph," an official said.
But some sections feel that given the astronomical costs involved, the acquisition is unjustified - particularly since the railways can work on an alternative model at a fraction of the costs.
"The LHB-design coaches (certified for 160 kmph speed) can be upgraded to 180-200 kmph to create a train set with two locos at each end... This train set may not resemble an Inter City Express or a Shinkansen, but will more than serve the purpose," officials said.