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 user Bill Cob: "...the lack of synchronization is tied to revs per mile. They are slightly different. Either one has slipped somewhere (as in a turn), or the tires on the drive wheels are slightly different in diameter. By 'tires', I mean the steel rings sweated onto the wheel rims. It's no big deal or concern. If there was a weight difference between the engines, then the tire or resulting rail deflection would cause a phase change, too. Slip is the usual culprit."

Q~Are they oil burners?
A~All UP Steam Program locomotives were converted to use No. 5 fuel oil. They originally burned coal.

Q~Were there a bunch a people?
A~Oh yes. Places were packed along the tracks. My estimate was around 2,000 people, although I figure there were actually more!

~E: Better video taken in the hot seat up:    • Union Pacific 4014 & 844 doubleheader...  

(Yes this the guy leaning out the window in those photos lmao)