The tension in the couplings are the same. The car is on level ground and therefore there is the same amount of force being exerted on each car by the pulling of the train. I'm not sure exactly, though. It would seem that there would be greater force on the trains near the front, because there are trains behind them exerting a pulling force. And, it seems like it would be easier to remove a train near the back while it's moving than a train in the front.

According to Newton's third law every action will have an equal and opposite reaction. Here the action is the locomotive pulling the first car. An opposite and equal reaction is had on the first coupling, linking the first two cars. That coupling then acts on the second car and thus the force exerted on the second car is equal to that of the first car. It continues like this down the line and hence the tension is the same in all the couplings.

The tension should be the same for all of the couplings because all of the cars are accelerating at the same rate. Because acceleration is directly proportional to force, or in this case tension, and the cars are all accelerating at the same rate there should be an equal stress on all the ties. The only way the tension could be different is if the ties are loose and the cars begin their acceleration at different times, but there is no indication that this is the case.

Since it is a level track, the tension in the couplings are the same anywhere along the train. If the train were travelling uphill, the tension would be greater between the locamotive and the first car, but considering that it is on a level track, the tension is the same between all the cars.

I would say the tension is the same anywahere along the track because the force exerted by the locomotive on the first car equals the force between each adjacent car or the cars, such as the caboose would shoot off in the other direction.