Here are 10 big developments in the case:
Sources say that Mr Kumar's wife handed over bags containing evidence such as investment documents to a railways inspector, who was allegedly directed to collect them by the IG.
The inspector reportedly met Mr Kumar's wife at ISKON temple and collected the bags and kept it at his home. Sources say he later panicked, moved the bags to his friend's house and tipped off the Mumbai branch of the CBI.
CBI then raided the cop's friend's house and found Mr Kumar's investment documents and jewellery.
Sources also say that more evidence allegedly pointing to Railway Minister Pawan Kumar Bansal's role in the case have emerged. Sources say the CBI is likely to interrogate him soon. Mr Bansal resigned as Railway Minister this evening. (Read)
Mr Bansal's nephew, Vijay Singla, was arrested last week for allegedly accepting Rs. 90 lakh from a senior Railways official, Mahesh Kumar, who reportedly wanted a "more lucrative" posting in the Railways.
CBI sources have told NDTV that Mr Kumar, who is in CBI custody now, met Mr Bansal's nephew at the Railway Minister's official residence in Delhi on April 7 along with a middleman. Mr Kumar has reportedly also told CBI that 10 days later on April 17, he also met the Railway Minister in Mumbai.
After the meeting on April 17, the middleman asked Mr Kumar to speak to the minister's nephew and gave him a phone number, sources in the CBI say. This number, they add, was the number of Mr Bansal's official residence in Delhi.
Mr Kumar has reportedly also told the CBI that he later negotiated the "deal" with Mr Bansal's nephew during several meetings held at the Railway Minister's house.
He has also alleged that Mr Bansal's nephew often used the rooms of the minister's aides in the rail ministry office. The CBI has reportedly already questioned Mr Bansal's personal secretary on why Mr Singla was allowed to use government office space.
Mr Bansal has denied any involvement in the bribery scam. In a statement released last week, he said he "had no knowledge or clue about the matter at all." He also insisted that his relatives "do not and cannot meddle in my official functions or influence my decisions."