Decided to try and attempt to replicate the train that claimed the World Speed Record for Steam for the Germans in 1936. I have a reasonably good German book 'Schnellverkehrs des Reichsbahnzeit' which went into the various aspects of high-speed travel of the time, including a quote from one of the gents who was there on the train - but it was abridged. In my excitement at finally getting somewhere with a record-breaking train, I did not notice that one of the passages was missing key words that would change the translation and meaning of the relevant bit about the carriages behind the big 05 002.
My book claimed that the regular test train (made of a dynamometer car and three/four of the newest long-distance, high-speed carriages) had been swapped out for one of the three-section diesel-electric railcars, and it was this combination that managed the 124.5mph on 11 May 1936.
Turns out this was WRONG. The regular test-train on this particular day was made up of the dynamometer and three carriages, and the loco crew got the bit between their teeth IN RESPONSE to one of the railcars pushing 125mph!!!
Details and appropriate coaches are now being sought out.