According to Dengeki Online, the second season of Toaru Kagaku no Railgun was announced at the Dengeki 20th anniversary festival. Details of the new season will be revealed in the Monthly Comic Daioh December issue, to be released on October 27th.
Redfoxoffire said: I don't get why people wanted Index 3, Railgun was always better. I'm happy.
I prefer Railgun too, and I think its a bit unfair to ask for a new season of Index already, since the 2nd season was more recent than Railgun. They'll likely go back and forth, since both of these franchises are some of JC Staffs biggest money makers (Railgun moreso)