When setting hand brakes on cars of a ten-car train laid up on a grade, which cars should the operator choose?

In the context of setting hand brakes on cars of a train laid up on a grade, it is essential to consider the principle of gravity and the distribution of weight within the train. The correct choice involves the rear cars, which are generally the last cars in the train.

Choosing the rear cars, in this case, cars 8, 9, and 10, allows for the most effective use of hand brakes to prevent the train from rolling downhill. When the train is on a grade, heavier components and the shift in weight distribution can potentially cause movement. Applying brakes on the rear cars provides the necessary counterbalance to maintain the train's stationary position.

Setting the hand brakes on the end of the train is a standard safety practice, as these brakes will help to hold the entire train in place against the force of gravity. This positioning mitigates the risk of a runaway train scenario.

The other options do not provide the same level of safety because they focus on cars closer to the front or middle of the train, which may not effectively prevent movement without considering the gravitational forces acting on the entire train.