I do not know if it is true; I am simply conjecturing here.
What could have been done by such a machine downhill can only be conjectured, for the maximum speed was restricted rigidly to 80 m.p.h.1952 December, 'Mercury', “Modern French Locomotive Performance”, in Railway Magazine, page 804:
2
To infer on slight evidence; to guess at.
February 22, 1685, Robert South, All Contingences under the Direction of God's Providence (sermon preached at Westminster Abbey)
"Most likely he who is the shorter of the two;—no! he goes in the first, and is, of course, the eldest; but we have no time for conjecturing now." Human reason can then, at the best, but conjecture what will be.1838 (date written), L[etitia] E[lizabeth] L[andon], chapter XIX, in Lady Anne Granard; or, Keeping up Appearances. […], volume I, London: Henry Colburn, […], published 1842, →OCLC, page 243: