What is the first step to take if it is necessary to cut out a door that is stuck open on a revenue train?

In the situation where a door on a revenue train is stuck open, the first and critical step is to close the cut-out cock on the air line to the door engine. This action is essential because it helps to isolate the air supply to the door mechanism, preventing any unintended operation or movement while troubleshooting the issue. By cutting off the air, you also enhance safety for both passengers and crew, as a stuck door could pose a risk during transit or make it difficult to manage passenger boarding and alighting.

Once this step is completed, other actions, such as notifying the conductor or checking for obstructions, can be taken to address the stuck door situation in a safe and orderly manner. However, ensuring that the door mechanism is frozen in its current state by cutting off air supply is of utmost priority to avoid accidents or further complications.