What is the signal series in Tunnel A?

The signal series in Tunnel A refers to a specific set of signal aspects that are utilized for controlling train movements within that tunnel. The choice of "100 even" indicates a foundational signal series, which typically handles basic train operations and assists in ensuring safety through signaling.

In a railroad context, signal series often categorizes different aspects of signaling that convey vital information to train operators. The "100" series would typically pertain to standard speed and movement expectations under usual operating conditions, thus making it a common selection in controlled environments like tunnels. The designation of "even" signifies that this series is part of a standard system without any variations or special conditions that might involve odd-numbered signals or unique aspects.

In the context of Tunnel A, signaling systems need to be clear and consistent to maintain safe operations, and the selection of the 100 series is likely aimed at enhancing clarity and reducing the risk of confusion due to complex signaling that might be posed by higher signal series numbers.