Which of the following describes a portal?

The description of a portal as the locations where trains enter and exit tunnels is accurate because it captures the specific structural feature associated with the transition between open air and enclosed rail passage. A portal typically refers to the openings of tunnels where trains move from surface tracks into a tunnel system, and vice versa. This is vital for roadway worker protection, as these areas can present unique hazards, such as restricted visibility, sudden changes in environment, and the necessity for appropriate communication and signaling measures to ensure worker safety. Understanding what constitutes a portal is essential for training workers to recognize hazardous zones and adopt the necessary caution when operating near such locations.