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 flip over, lose all of their items, and lose a lot of speed as well. Unlike cars, buses, or trucks, Wiggler Wagons cannot be launched into the air using Stars, Bob-ombs, or Bowser Shells. Wiggler Wagons also wobble around noticeably when they drive.
Each Wiggler Wagon varies in length, with the one at Mushroom Bridge being ten body segments long in single-screen races, while being five body segments long in split-screen races. The one at Mushroom City is always six body segments long, regardless of how many screens there are.
In Mushroom Bridge, the Wiggler Wagon is the slowest vehicle in its lane, regardless of how many players are racing, while in Mushroom City, the Wiggler Wagon is the fastest vehicle in its lane on the lower level.
Originally, there was going to be a second Wiggler Wagon in Mushroom City, but it was removed in the final version of the game.