5 Raw minutes of pacing UP 4014 through Echo Canyon

I am at a loss for words... We paced a Big Boy... Q&A Q~How fast were they going/What was your speed? A~35-40 MPH (56KPH-64KPH) Q~Did you have a shotgun mic on your camera? A~Believe it or not I didn’t! I shot this with my phone of of things! Q~I see that the A (Front) engine on Big Boy leads the B engine by about 40 - 60 degrees at the start of the video. At the end of the video, both engines are in phase. What is causing this (slip or a slight average rolling radius difference ? Any ideas ? A~From u