When should blocks of empty cars be placed at the rear of a train?

The correct answer is based on the importance of train handling and safety practices regarding the placement of empty cars. When there are 30 or more empty cars in a train, placing them at the rear helps to ensure better stability and control over the train's movements.

This practice minimizes the risk of the train becoming unstable, especially during maneuvers such as accelerations and decelerations, which can lead to a phenomenon known as "string lining." A heavier load at the front helps to push the lighter empty cars, allowing for smoother operation and maintaining the desired braking characteristics of the train.

Additionally, positioning a significant number of empty cars at the rear helps in distributing the weight more evenly throughout the train, making it easier to manage both operational and safety aspects. These factors are especially critical for longer trains where the dynamics can change dramatically depending on cargo weight distribution.

Thus, the guideline specifically noting 30 or more empty cars ensures that the train remains manageable and can safely navigate its route.