How Efficient Were the "Big Three"?

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Greetings,

In the rudimentary research I've done on N&W's big three (J, A, Y6/a/b),
I've gotten the impression that they were among if not the most efficient
steam locomotives built in the United States, in some cases outperforming
the first-generation diesels. But exactly how efficient were they? From
what I've heard the maximum fuel efficiency of a steam locomotive is about
15%. Did any of the big three have the abil

Eli Santinity to reach this efficiency level?
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