Afaik the breaking capability of cargo trains is lower because they use simpler brakes and only use the airline for breaking, which limits the speed at which you can apply the brakes on a long train without risking breakage of the links. Passenger trains use disk brakes and through electronic control can apply the brakes evenly across the entire length of the train, so the brakes can be applied much more quickly.

I think the weight of a car itself wouldn't change the theoretically possible stopping distance, since the momentum is proportional to the mass, but so is the friction between wheel and track, so mass cancels out.