I know there is a discussion of this in some thread but I’m not finding what I’m searching for so here’s the question. For locomotives designed for speed, does running at 40 mph have a negative impact on fuel and/or water consumption? (I’m thinking the effect is de minimis.) I ask because 844, 4449, 2926, 251, 614, 611 and so on were built to race.

I don't know if this helps, but 611 was built to run at 40mph with a 15 car passenger train anywhere on the N&W system.

I'm recalling a bit here that there was a reply to some thread that described steam locomotives as heat machines. The explanation given went into how a steam locomotive will pop into zone where the work required of the loco creates the optimum heat, optimum steaming, and optimum output. Just curious if the listed Northerns (other than 611) running at a ceiling speed of 40 mph is below the locos "optimum zone".

The rule of thumb I have read is that maximum drawbar efficiency is achieved at the same speed that maximum drawbar horsepower is achieved.
The answer to the question might be in David Wardale's Red Devil book.

Tom Hamilton