The most recent Atlas OEM Titan firmware supports Horn Master.
To find out if the firmware in your locomotive supports Horn Master,
have the decoder speak out (or read back in service mode) the CV55.1.SI CV's (Horn Type, Alt Horn Type, Horn Reverb Amount, Horn Reverb Distance) or the CV130.PI.SI User-defined Horn Component Pitch CV's.
If the decoder speaks out the CV number only (or the service mode readback fails), the firmware does not
support Horn Master.