On an average day, as many as 35,000 tickets in various classes of all trains that originate from Delhi or cross the city are reserved under the emergency quota which is also known as VIP or Headquarters quota.
The agency suspects that certain vested interests within various unions of railways and in alleged conspiracy with private travel operators confirm these tickets against a premium, official sources said here.
Initially, the probe may be limited to the national capital but, if required, it may cover other states, they said.
CBI has already registered a Preliminary Enquiry (PE) last week against unknown persons to carry out a thorough probe for misuse of the railway tickets issued under quota to various railway unions.
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The quota is meant for emergencies which includes medical necessity but the agency has alleged that the process was abused by certain vested interests.
It had found some related documents while looking into the functioning of railways as part of its probe into the alleged bribery case involving the nephew of former railway minister Pawan Kumar Bansal.
CBI had, during a discreet probe, claimed to have found that the emergency quota tickets to unions were being handed over to some travel agents who used to charge a huge price for confirming a wait-listed ticket.
A senior CBI official claimed it was a full-fledged racket.
The official said the travel agents had been identified and would be called for examination soon besides some of the office bearers of a few railway unions who had allowed confirmation of wait listed tickets.