What is the minimum headway in one direction during single-tracking in an under-river tunnel when the running time between two towers is 3(1/2) minutes?

To determine the minimum headway in one direction during single-tracking in an under-river tunnel, we need to consider the running time between the two towers, which is given as 3.5 minutes. Headway is the minimum time interval between two trains traveling in the same direction on the same track, ensuring safe operation and allowing for any necessary turnaround or clearance times.

In a single-track scenario, trains must alternate their passage through the track segment. The minimum headway must account for the time it takes for a train to clear the section before the next train can enter.

Typically, in practice, the headway is longer than just the running time to accommodate various operational factors, such as acceleration, deceleration, and station handling times. A common calculation adds a buffer to account for safe spacing, resulting in a total minimum headway that needs to be greater than the mere running time.

In this case, with a running time of 3.5 minutes, adding a reasonable buffer for safety and operational efficiency leads to a minimum headway of 7 minutes. This allows for adequate spacing between trains to prevent potential conflicts and ensures smooth operation in the confined space of a single-track under-river tunnel. Hence, the correct choice reflects these