Prior to coupling a Siemens to a Comeng, what must be done on the Siemens?

The correct action prior to coupling a Siemens to a Comeng is to turn the towing cock. This step is essential because the towing cock allows for the connection of pneumatic systems between the two units. By turning the towing cock, you ensure that the necessary air supply is available for the brakes and other systems to function correctly once the units are coupled. This is a crucial safety measure to prevent incidents that could arise from improper air pressure or braking functions when the trains are in operation.

The other actions listed, such as releasing the brakes or activating emergency lights, while also important in the general context of train operation, do not directly address the immediate requirement for coupling the two trains in question. Thus, focusing on the proper operation of the towing cock is paramount for safe and effective coupling.