So, not all points of the moving wheels of the carts are moved equally quickly. What part of the rolling wheel just moves slower?
It is easy to think that slower move all the points of the wheels that are currently in contact with the ground. Strictly speaking, at the moment of contact with the soil of these wheels perfectly still.
All of the above is true only for the wheels rolling, and not for such, which rotates on a fixed axis. In flight the wheel, for example, the top n bottom of the rim are moving with the same speed.