The examination was to be conducted for 1197 vacancies in paramedical category and 162 vacancies in technical research supervisor category across 19 RRB centres.
"The examination for July 25 has been cancelled as a new full-proof system for conduct of the examination is yet to be put in place," said a Railway official.
He said several checks and counter-checks are being firmed up to ensure confidentiality in the examination system while technical audit is still underway in the RRB centres.
CBI had on June 18 claimed to have unearthed the recruitment scam and arrested eight persons, including son of Chairman of Railway Recruitment Board, Mumbai, for allegedly leaking exam paper for money following which examinations scheduled for June 27 was postponed.
Describing the unearthing of the alleged scam as a big blow to its efforts to fill up vital posts, Chairman Railway Board Vivek Sahai had said they would adopt "checks and balances" in the examination procedure to prevent leakage of question papers.
Railways, which have about two lakh vacancies, started the recruitment exercise for filling about 40,000 posts in phases on June six.