Why is there an diesel engine at the rear of a train?

Answer 1

The most common reason is it provides additional pushing power for the train. Advanced answer is that certain trains have remote control locomotives at the rear (helpers) or in the middle (swing helpers) that can push the train up hill and the use dynamic brakes on down hills to slow the train. By doing this it saves fuel and brake shoes costing the railroad less and keeps air in the train's brake line by cutting down on brake applications thus making the train safer.