Final answer:
The maximum speed for a work train on a straight track is not specified; however, average speed calculations and principles of motion such as relative velocity and acceleration-deceleration are important concepts in physics.
Step-by-step explanation:
The maximum speed allowed for a work train on a straight track isn't provided in the background information given, as that value can vary depending on safety regulations, engineering limits, and track conditions. However, when calculating the average speed of a train that travels 40 miles one way and 40 miles back, the total distance would be 80 miles. The average speed would be calculated by dividing the total distance by the total time it took to travel that distance.
For the other provided physics scenarios, such as the speed of a man relative to a moving train, and a light-rail commuter train's acceleration and deceleration rates, motion principles such as relative velocity, acceleration, and deceleration come into play which are integral parts of Classical Mechanics, a branch of Physics.