How many locomotives may be coupled for multiple-unit operation without exceeding regulations?

The correct number of locomotives that may be coupled for multiple-unit operation in most commonly followed regulations is indeed 12. This limit is set to ensure that operational efficiency is maintained while preventing excessive strain on the train handling systems and air brake capacities. Keeping the number of locomotives within this limit helps in managing factors such as brake distance and overall train dynamics.

When the number of locomotives surpasses this threshold, factors such as increased stopping distances and more complex operational considerations come into play, which could potentially compromise safety and performance. Thus, the regulation serves to standardize locomotive configurations for optimal reliability and safety during operation.