Today I opened the UP844 and its tender on Pacific Surfliner. While climbing up a 0.5% grade, the UP844 hit 127 miles per hour, and continued to gain speed until it was going 137 miles per hour. It maintained this speed for about two minutes. Mallard's speed record is beaten once and for all!!! (Not really R.I.P.)
It's an impressive achievement, going that fast with most steam engines is hard. Just for sake of fairness though, I've achieved speeds in excess of 127mph with Mallard itself in Train simulator. I can't recall exactly how fast but I think it may have been somewhere in the 130s. Of course, with a nice electric one can go even faster... although I've never been above around 142mph or so!
