The product of (mass) x (acceleration) is equal to (force).
So if (force) doesn't change and (mass) changes, then
(acceleration) has to change in the other direction from (mass)
in order to keep their product constant.
If the (forces) on both wagons are equal but one wagon has
double the (mass) of the other, then the more massive wagon
has (1/double) = 1/2 the acceleration of the less massive one.
