From the Super Mario Wiki
Wiggler Wagons are vehicles found in Mario Kart: Double Dash!! They look like mechanical versions of the green Wiggler found in Super Mario Sunshine. They only appear in the Mushroom Bridge and Mushroom City race courses as moving obstacles. If a racer hits one of these long vehicles, they will flip over, lose all of their items, and lose a lot of speed as well.
Each Wiggler Wagon varies in length, with the one at Mushroom Bridge being ten body segments long in single player races; it's only five body segments long in a multiplayer race. The one at Mushroom City is six body segments long.
Originally, there was going to be a second Wiggler Wagon at Mushroom City, but it was removed in the final version of the game.