the tension in a coupling describes the force which is necessary to pull all following waggons.
to a) The coupling between the 30th and the 31rst waggon has to pull 20 waggons.
You are supposed to know how to calculate the force if you know the mass and the acceleration: . Thus:
to b) Now the coupling has to pull only one waggon: that means:
A few personal remarks:
i) A train waggon with a mass of only 7 t must be empty.
ii) The electrical lokomotive E103 which was build to pull (relatively light) passenger waggons has an initial pulling force of 312 kN. So either I made an ugly mistake or the problem isn't very realistic.