According to Newton's 3rd law, the tension between the engine and the 1st freight cars would equals to the force required to pull all the freight cars from the 1st one on ward. In addition, Newton's 2nd law states that the F = ma, which means that force is proportional to the mass of all the freight cars, given that the freight train is accelerating. So if some of the cargo in the last car were transferred to other cars, their total mass would stay the same, and so as force F and tension.
