What is a critical consideration when adding locomotives to a consist?

When adding locomotives to a consist, one of the most critical considerations is brake testing requirements. This is essential because each locomotive in the train contributes to the overall braking performance and safety of the entire train. Each unit must be thoroughly checked to ensure it is functioning properly within the overall braking system, meeting regulatory standards and safety protocols. Proper brake testing ensures that all locomotives can effectively stop the train in a controlled manner, preventing potential accidents or malfunctions that could occur if one or more locomotives were not adequately tested or functioning.

In this context, the other considerations like cost efficiency and fuel efficiency, while important in operational decisions, do not take precedence over the safety and compliance implications dictated by brake testing requirements. Last minute changes can disrupt planning but do not inherently address the safety concerns associated with the brake system, which must be monitored closely to maintain the integrity and safety of the train.