I think that the tension in the couplings between the train cars would be the least between the last car and the caboose. The amount of tension is largest against the front of the train and continually decreases. The train cars in the back have the aiding forces and momentum that pull the car with less tension then the front car.

Because of Newtons law F=ma, the acceleration of all cars is equal, however the mass of what is being pulled decreases as you get to the cars closer to the caboose. therefore the forece of tension (F) decreases and is least at the couplings between the last car and the caboose

all of the cars because they are linked have the same velocity and the same acceleration. the only thing that can vary is the mass. start at the locomotive, it is moving foward at the same accleration and velocity as the entire train but it also has a force exerted back on it by the rest of the train. So now if you go to the caboose it is being pulled at the same accleration but does not have the mass of the entire train bhind it pulling back. Therefore the tension is the lowest between the last car and the caboose