This demonstration has been successfully run in the lab, but never before performed before a class. The main things to watch out for that this initial attempt revealed are:
(1) The track should be levelled in both directions, so the carts are neither leaning to one side nor accelerating back and forth.
(2) The most impressive way to do the demonstration is also the most difficult, making a light cart collide with a heavy cart. This will cause the light cart to rebound back through the first photogate. This can show that the momentum of one cart can actually come out higher than the momentum of the system, but when added to the momentum vector of the first cart which has rebounded, the system's momentum is conserved. .
In order to do this, the combination of masses and anything placed between the carts to help them rebound must be carefully considered and tried out beforehand. The best combination yet discovered is to use the bearing cart labelled "collision cart" for its magnetic ends which can attach a stiff spring (one from a diesel engine masses 91 grams) and an extra 600 grams (bringing the total mass to about 1200 grams). The specific spring may be difficult to locate, and the experiment must be tested beforehand. The point of it is to get the smaller cart to rebound with enough velocity. As the momentum of the small cart decreases (after the collision) the error in the experiment may become large.