Final answer:
When a train is moving up a steep grade at a constant velocity, the forces acting on the train are gravity and air resistance. The power required to maintain this velocity is equal to the power expended against gravity and air resistance.
Step-by-step explanation:
When a train is moving up a steep grade at a constant velocity, there are two main forces acting on the train: gravity and air resistance.
In order to maintain this constant velocity, the power supplied by the train's engine must be equal to the power expended against gravity and air resistance.
For example, if 75% of the power is used to overcome gravity, which equals mg*sin(theta) where theta is the angle of the incline, then the remaining 25% of the power is used to overcome air resistance and friction.