Electronically Controlled Pneumatic (ECP) brake technology can improve both safety and braking performance of trains. One of the technical barriers is the interchange requirement of a freight car. This barrier can be addressed either by an overlay ECP or an ECP emulator. Under a previous effort, Sharma & Associates (SA) procured three sets of emulator car brake equipment from Zeftron. Hardware and software modifications were identified and implemented to the system to comply with S-4200, including limited fault testing using laboratory setting. This effort reports the interoperability compliance testing of the emulator under field conditions with other ECP industry vendor’s locomotive and freight car equipment. Initialization procedures, including car sequencing and train building, were completed under the ECP mode. Graduated application and release tests were performed in RUN and SWITCH and CUTOUT modes. Subsequently, fault tests were conducted as prescribed in S-4200. Pneumatic tests were carried out under emulation mode. The testing revealed minor issues relative to dynamic addressing, sequencing, and low battery detection. Appropriate software modifications were made and tested in the laboratory confirming that the emulator Car Control Device (CCD) passed all required tests. Researchers recommend that the emulator system be evaluated for environmental compliance.