Starting December 2016, you would need to have an aadhar id to book train tickets – for both reserved and unreserved categories; besides holding true for online as well as counter tickets.
The benefits are huge – this will certainly bring down illegal/unauthorized bookings by agents who often game the system. But it will also make the entire process very cumbersome (what about foreign travellers?).
IRCTC Launches Travel Insurance
IRCTC is hopeful that with launch of Aadhar linked ticketing services in all booking categories, even unreserved, will boost its travel insurance service. As of now railways insurance service is limited to online reserved ticketing and with Aadhar, it can be extended to unreserved category as railways will have access to personal data of passenger in unreserved category (source).