How does the presence of distributed power (DP) units affect the procedure for conducting an air brake test on a long train, and what specific considerations must be addressed?

Distributed power (DP) units, which are locomotives placed within the train consist, typically controlled remotely from the lead locomotive, significantly impact the air brake test procedure on a long train. The presence of DP units creates multiple air sources within the train, altering the timing and dynamics of air brake application and release, and requiring adjustments to the standard air brake test procedure. When performing an air brake test on a train with DP units, it is crucial to first establish communication between the lead locomotive and the DP units. This ensures that the DP units are properly configured and are responding to commands from the lead locomotive. It's essential to verify that the DP units are online and functioning correctly before beginning the air brake test. The initial brake pipe leakage test must be conducted with all locomotives contributing air to the brake pipe. The presence of DP locomotives shortens the time it takes to charge the brake pipe, but it is still essential to ensure that the brake pipe is fully charged to the required pressure before proceeding with the test. After charging the brake pipe, the brake pipe leakage test is performed to ensure that the air leakage is within acceptable limits. The presence of DP units can make it more difficult to detect leaks, as the multiple air sources may mask small leaks. For this reason, it is important to pay close attention to the brake pipe pressure during the leakage test and to look for any signs of excessive leakage. The next step is to make a brake application. The brake application is made from the lead locomotive, and the DP units should automatically respond to the brake signal. It is essential to verify that the brakes are applying on all of the locomotives, including the DP units. This can be done by observing the brake cylinder pressure on each locomotive. A walking inspection of the train is required. Because DP units effectively create multiple trains within a train, it is imperative to confirm that the brakes apply and release correctly on cars both ahead of and behind the DP units. This ensures that the DP units are not causing any abnormal slack action or other problems. The effectiveness of the rear end brakes must be verified. With DP, the rear of the train will respond more quickly to brake applications and releases. Special attention must be given to ensuring that the rear brakes are functioning correctly and that there are no excessive delays in brake application or release. Communication is essential throughout the air brake test. The crew members at the head end and the DP units must communicate frequently to ensure that all steps are being performed correctly and that any problems are being identified and addressed promptly. The specific considerations to be addressed when performing an air brake test on a train with DP units include: ensuring that the DP units are properly configured and communicating with the lead locomotive; verifying that the brakes are applying and releasing correctly on all of the locomotives, including the DP units; checking for excessive brake pipe leakage, taking into account the presence of multiple air sources; paying close attention to the rear end brakes and ensuring that they are functioning correctly; and maintaining clear communication between all crew members involved in the test. Failure to address these considerations can lead to inaccurate air brake tests and increase the risk of accidents.