After making a brake pipe reduction when adding helpers, what should be done next?

After making a brake pipe reduction when adding helpers, cutting out the automatic brake valve is the appropriate subsequent action. This procedure is essential because it ensures that the automatic brake system does not interfere with the operation of the train after adding helpers. By cutting out the automatic brake valve, you effectively isolate the brake system to allow for proper control and coordination among the locomotives and the train, ensuring that the added power from helpers does not inadvertently cause unintended brake applications or complications with the braking system.

The other options involve steps that might be relevant in different scenarios or contexts, but they do not align with the procedural focus required after making a brake pipe reduction in the context of adding helpers. For instance, opening the brake pipe angle cock might be necessary later but is not immediately relevant following the brake reduction. Testing the ETD system or closing the throttle also pertains to different operational adjustments and does not address the immediate need to cut out the automatic brake valve, which prioritizes safe and efficient train handling.