Personally i think this is one of the best episodes of the second series. Creepy children are always a plus in my opinion and they may not get much creepier than the boy in this ep! So basically an evil spirit or the devil (i'm not sure) has posessed a little boy and his twin who died at birth has come back to seek revenge and kill everyone. Well from the very beginning with the pink balloon luring the boy infront of the train this episode stood out as a very dark episode, and the exorcism scene is defietley the highlight along with Scully being thrown around a room by the spirit of the little boys dead twin!
The x files at its creepy best, 10/10 for creep factor, 10/10 for the episode as a whole!