If there were no net force, the cart's velocity would be constant.
In considering all the cases, if we ignore the frictional forces, then momentum will be conserved for every case. This does not mean that kinetic energy must be conserved. For example, imagine equal mass carts colliding head-on and sticking together. Before the collision, the two carts, in this case, have equal and opposite momentum, so the total momentum before the collision is zero. After the collision they are stuck together and not moving, so the total momentum after the collision is still zero, as expected. Kinetic energy, however, has changed before and after the collision. Before the collision both carts were moving, so there was kinetic energy before the collision. (Remember, kinetic energy is not a vector, KE=1/2mv2.) And after the collision, since there is no motion, the kinetic energy is zero.
Kinetic energy can be lost in other types of collisions, too. ("Lost" means that is is converted to other forms of energy.) But, as long as there are no net forces acting on a system, even though kinetic energy may be lost, momentum is still conserved.