I read on this forum that the drivers on 3751 slipped on this trip, and there was some criticism about it. Difficult to hear well, but I hear wheel slip right after the 1:15 mark.
At the 13:00 mark, you see them go full blast into a tunnel. When they come out, everyone is coughing and the diesels are totally soot covered.
His description --
"Video picks up with 3751 on the Caliente, CA curve and takes you through famed Tunnel 5 in real time. Tunnel 5 is where the Northern allegedly slipped, steam cleaning the roof of the tunnel and depositing soot all over the engines, first few cars and lucky vestibule riders."