E-ticketing facility allows booking of tickets by oneself at home or office and at any time during the day or night, in general. Most of the tickets are sold this way.
The monetary transaction occurs using automatic money transfer from one account to another. This is done with a receipt and is recorded. There is no human agent coming in between to charge a bribe. Hence there is no corruption, in most cases.
For people who have not the facility at their locality or village, the situation can be different. Further, e-ticketing facility is a transparent process, automatically decided by the computer and the reservation software. Hence no individual makes a decision and can charge no bribe.
Since people can reserve more comfortably at their convenience, and many people are computer literate now in India, very few persons go to reservation counters to stand in the queues and make reservations. Since that facility is automatic and computer based, the clerical staff or any other agent cannot charge extra, as an exchange for doing any favour to the passenger. Superseding another passenger does not happen, hence this is another reason for no possibility of bribes.