What must not happen while operating a Brake pipe rescue?

When performing a Brake pipe rescue, it is essential that the trains remain electrically uncoupled to ensure safety and proper functionality of the brake systems. If the trains are electrically coupled, it could lead to unintended activation of systems that could interfere with the brake operations, potentially causing a hazardous situation. Keeping the trains electrically isolated prevents any unwanted electrical interactions, ensuring that the brake systems of each train operate independently and as intended.

The other options suggest actions or conditions that could compromise safety and effectiveness during the rescue operation, but the necessity of keeping the trains electrically uncoupled is paramount to maintaining control and ensuring the integrity of the braking system during this critical operation.