This one is closer to novel character's look and feel was supposed to be. People out there easily forgets many Toaru characters are still kids (Saint Seiya "I'm 16yo, but I looks like 21-24yo" syndrome). I guess there is only other artist who draws Misaka and Misaki as they were supposed to be (loli and oppai loli (?)).
The artist is Hisasi. Originally from the 2011 artbook "Toaru Kagaku no Railgun" SPECIAL, sold as a special issue of Dengeki Daioh magazine (the one that sells the original manga).