They are orange and security carts scattered in the Underground Tunnels as well as in front of the Fortune City Hotel. These carts can kill around 60-70 zombies before breaking down, releasing a puff of smoke.
The yellow carts can hold 1 passenger along with the driver while the security carts with lights on the top has seats 2 seats in the back allowing a total of 3 passengers and the driver to use the cart.
The 4x4 has two names in the PC file, items.txt: IndustrialCart and SecurityCart. They are actually two distinct different vehicles, one is not a sub-vehicle of another. There is also the Postal Cart, another separate entry, which is only seen in Mail Order Zombrex, that although it cannot be driven in the game, it is set up to be driven in the PC files.
The only differences between the three vehicles: