[EricH]

There are few rules governing carts. The ones that are there are mainly practical limitations, as follows:

Originally Posted by Administrative Manual, Section 4.3.2

 Carts must remain in the team pit area when not in use for robot transportation;
 All carts should fit through a standard 30-inch door;
 Wheels on the cart must not damage site flooring; and
 Do not add music or other sound devices to the cart.

Nothing about number of wheels. (Also note--carts may need to traverse cable protectors/slightly uneven flooring.)

I would also add that height should be considered in the "fit through a standard door" requirement.