Who said that Haygood used the radio on his own motorcycle? By 12:35, both Harkness and King had their three-wheelers on top of the rail yard. One of them is seen riding around in the rail yard in the Hughes film. He could have just borrowed their radios
After leaving the RR yard Murray stood on the grassy knoll and photographed the crowd gathered on N Elm, including the area near the parked motorcycle of Clyde Haygood. His helmet is visible in the photo near his bike. Murray also caught the north peristyle wall with an advancing shadow moving southward about 3-4” per minute. Modeling this wall shadow allows an estimation of the time as 12:36. My original posting on this technique can be found in this forum here.https://www.jfkassassinationforum.com/index.php/topic,2013.msg54679.html#google_vignette