What action should be taken if brake pipe pressure falls below 90 psi?

When brake pipe pressure falls below 90 psi, it indicates a potential problem with the braking system that could compromise the safety of the train operation. Therefore, treating the brake as a rupture if braking power is insufficient is critical to ensure safety.

In this situation, the engineer must assess the braking capability immediately. If there is insufficient braking power, the train could be at risk of not being able to stop effectively, which is a serious concern in rail operations. By treating it as a rupture, the engineer takes precautionary measures to mitigate risks associated with the brakes potentially failing completely.

While notifying the central dispatch office and other actions might seem prudent in certain circumstances, the priority here is to address and manage the immediate risk to the train's operation and ensure that it can be stopped safely. Increasing speed to maintain momentum is contrary to the necessary response when an issue with the brakes is detected. Therefore, option C is the most responsible and safety-conscious action to take.