Rail Dupe

I have searched this and found many similar dupes, but this one relies on powered rails bedside the hole in the dupe machine. This dupe can dupe only powered rails.
It works on survival and adventure and creative (tho creative doesnt count).
Also the obsidian block seen, is just being used as a block that cant be pushed.

I thought it was a dupe in 13w10b but I was in Creative (derp = me) not a dupe in survival. It only affects 1.4.7 in survival it turns out.