I wish we did stuff like this in Britain. I'd love to see an unrestricted sprint down Stoke Bank between a Duchess and an A4. Then we could finally see which is fastest.
The A4 would probably win. Only because they were built for flat gradients. Duchess, however, would win the race on the WCML.
It's more nuanced than that. When an A4 had runs up and down the WCML for the 1948 locomotive exchanges it didn't do noticeably better or worse than a Duchess. It's generally agreed that the A4s were more free running and the fastest engines on the ECML, but they weren't the most powerful. I recall reading that Eastern enginemen preferred Gresley Pacifics for the named expresses with lighter loads and higher average speeds, but if they had to take a heavy load of 10+ coaches out of King's Cross they preferred the Peppercorn engines. The Duchesses on the other hand are definitely more powerful than an A4. They have a noticeably higher tractive effort and hold the record for highest horsepower output out of any British express engine. They have very few recorded instances of speeds in excess of 100mph thoug, but that's largely because of the WCML's characteristics. It's worth mentioning that a streamlined Duchess is actually more aerodynamic than an A4. It would make no difference in normal running, but when flying down 5 miles of 1 in 200 track it could be crucial. I think you're right that on a point to point race on their respective mainlines, each engine would win on home turf. But I reckon a record attempt down Stoke would be pretty close.
