What should be done with the angle cock on the car that is to be detached at McPherson?

Closing the angle cock on the car that is to be detached at McPherson is the correct action because it prevents air from escaping the brake system of the car. By closing the angle cock, you ensure that the remaining cars in the train maintain sufficient air pressure to operate their brakes effectively. This is crucial for safety, as an open angle cock can lead to a loss of air pressure in the entire train line, potentially causing an emergency situation where the brakes would not function as intended.

Additionally, closing the angle cock helps prevent dirt and debris from entering the brake system when the car is removed. It is a standard procedure in train operations to manage the integrity of the train's braking system and ensure that other cars remain operational after the detachment. The other options, such as leaving it open or removing it, could compromise the safety and functionality of the brake system. Sealing it is unnecessary and would not serve the same protective purpose as simply closing it.