Rlsteam (Richard Leonard)--
I ***LIKE*** your "as never were's"! I've fantasized about some of them, and have one quibble: I think the PRR classes for their Hudsons, Northerns (and Challengers) would have been P1, R5 (and GG-5): going by the example of their steam L-1 and electric L-5, I think they had the idea that the class numbers would skip to -5 for an electric with the same wheel arrangement as the -1. P5 would have been given that designation when it was still thought conceivable that they MIGHT get a steam 4-6-4 (which I'm sure they wouldn't have wanted to call a "Hudson"!). A few years later they decided to start the series with the electrics (R-1 and GG-1), but if I had to guess I'd predict that the PRR motive power department was traditional enough that they would have wanted to leave the same gap if they had later bought steam R or GG types. (How's that for nit-picking?)
In the 1980s a book was published of paintings of proposed but un-built British locomotives, including an LMS 4-8-4-- sorry, don't remember author or title. The American philosopher David Lewis was also a devoted railway fan, and a member of the Great Western Society: when he published a book about "possible worlds" and the logic of counterfactuals (in which he used examples from railway history), he wanted to use the painting of the Great Western's unbuilt Pacific of the late 1940s (design drawn up just before the railways were nationalized) as cover art, but his publishers wouldn't let him. Sob.
...There is at least one un-faked photo of a New York Central Niagara without smokelifters, I think in one of Stauffer's books: the smokelifters of one Niagara were damaged in a minor accident, and the locomotive put back in service before they were replaced.