Bay Area Rapid Transit (BART) Practice Exam 2026 - Free BART Practice Questions and Study Guide

When the third rail trip is activated, the entire yard experiences a trip, which means that all operations within that yard are halted. This is a safety mechanism designed to address any potential hazards associated with the third rail system. The activation of the trip typically occurs in response to a situation that requires immediate intervention to prevent accidents or ensure the safety of personnel and equipment.

In this context, the entire yard trip ensures that all trains must come to a stop and that no further movement occurs until the issue causing the trip is resolved. This is crucial for maintaining safety standards and preventing any risks that could arise from an active third rail when it's in an abnormal state.

The other options, while related to yard operations, do not accurately describe the outcome of activating the third rail trip. For instance, only the mainline stopping would imply that yard operations could continue, which contradicts the nature of a trip. Disabling track switches or issuing a warning are also actions that might occur under certain conditions, but they do not encompass the primary response of stopping all yard activity altogether.