What component is essential for connecting a train to another train or car?

The component essential for connecting a train to another train or car is the coupler. Couplers are mechanical devices located at the ends of train cars that allow for the secure attachment and detachment of rail cars. They are designed to handle the heavy loads that trains carry, providing both strength and flexibility to accommodate movement during travel.

Couplers come in various designs, such as the Janney or knuckle coupler, which ensures a reliable connection that can withstand the forces acting on it during operation. This is crucial for safety and operational efficiency, as a faulty connection could lead to derailments or accidents.

In contrast, a drawhead, while related to couplers, typically refers to a specific part of the coupler mechanism that connects the car to the train. The running rail is part of the track structure that supports the train wheels, and signals are used for communication between trains and track control, playing different roles altogether. Hence, the coupler stands out as the key component for connecting trains and ensuring they operate as a cohesive unit.