Wiggler Buses (otherwise known as Wiggler Trains) are moving Wigglers in Mario Kart: Double Dash!! who appear in Mushroom City and Mushroom Bridge and bear a striking resemblance to tractor trailers. Should a player be unlucky enough to come into contact with a Wiggler Bus, the consequence of the contact will be the Kart flipping and the secondary racer (i.e. the non-racer) will be left holding on for dear life, yelling as if they had been hit with a Bob-omb. As they move - slowly - they make a noise, and the volume of that noise will increase upon incessant contact with the Wiggler Bus.
It is worth noting that starmen have no effect on them. However, it's also useful to know that if the player is driving a heavy vehicle, he or she can push other racers into the Wiggler Bus, and they might collide into the Wiggler Buses themselves.