The Fortnite patch notes, published by creators at Epic Games today, calls these “All Terrain Karts,” but I think that name might give them a little too much credit. They are, after all, just golf carts.
They’re not all that much faster than the shopping carts, and tip over quite easily. Although, I was using the Nintendo Switch while trying one out, so I was definitely more prone to over-correcting on the steering than a person playing on PC might be.
However, the carts are really quiet, which is a nice change from lots of other battle royale games, in which riding in a loud vehicle will immediately give your position away.