What is the primary duty of the equipment included in the train control apparatus?

Answer: B — To control train speed and display signals

The primary duty of the equipment included in the train control apparatus is to control train speed and display signals. This system is essential for ensuring safe train operations, as it regulates how fast a train can travel based on various factors such as track conditions, signaling information, and the presence of other trains on the same track. Functional aspects of the train control apparatus enable effective communication between the train and the signaling systems, which inform the conductor and crew of current speed limits and track conditions. By managing speed and interpreting signal information, the control apparatus plays a crucial role in preventing accidents and maintaining an efficient flow of train traffic throughout the rail network. Other options, while related to train operations, do not encompass the primary role of the train control apparatus. Track cleaning, passenger monitoring, and engine maintenance are managed through different systems or responsibilities within the railroad operation framework.