There aren’t many speed records that remain unbroken for the greater part of a century, but one of them is that of the fastest steam locomotive. As with so many such things, there’s a bit of controversy and more than one contender, but the one in the record books is the A4 Pacific, Mallard. In 1938, this locomotive thundered down an incline on the London & North Eastern Railway’s mainline in the north of England at 126 MPH. But can that number be taken as reliable? The Institute of Mechanical Engineers has a video in which they investigate.

It’s a fascinating look at the science of railway speed measurement as it existed in 1938, the record itself, and the paper dynamometer roll which recorded it. We’ve placed the video below the break, and in it, we see an in-depth analysis of the noise and inconsistencies in the recording, and see them come to the conclusion that a safer figure to quote would be 124 MPH.

Our assessment is that, of course, the LNER wanted to squeeze every morsel of publicity from it in a game of one-upmanship with their arch-rivals in the London Midland and Scottish railway, so it’s likely that their use of a momentary figure makes sense in that light. Even the best-laid 1930s jointed track would have been bumpy compared to modern continuous rail, and we are guessing that the ancient clerestory dynamometer car would hardly be as smooth-riding as a modern express coach. The achievement of measuring at all with mechanical instruments in such an environment at those speeds would have been tricky, to say the least. It leaves us wondering whether 1930s electronics could have produced some kind of trackside measurement device, but perhaps the LNER trusted their mechanical instruments more. Perhaps the Pennsylvania Railroad should have followed its example.

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