The line comes from asdfmovie, the long running animated sketch series by TomSka that built its humor on very short, random gags. The I Like Trains kid became one of its most quoted recurring bits, repeated across later episodes and endlessly clipped, remixed and reposted by fans. Its popularity comes from how self contained it is, with a setup and payoff in a few seconds that needs no context. That brevity helped it survive as a reaction sound long after the original sketches first spread online widely.