On Act 5, when you are on the train and press the very first button you see, the berserker will magically appear. It will be expressed through a cut scene where it busts through a couple of box, and a lot of objects "explode" from these boxes.

If you manage to make it around the berserker, go to the very last car on the train, (the one the berserker appears on,) You will find a cereal box, I believe to be called "Gears Crunch." On the backside of these boxes, there is a fictional prize you could win (which happens to be a small fluffy pillow!)