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More likely it has something to do with the grade of coal minedI have seen photos of C&I, C&EI, MONON (composite types), WABASH,
along the N&W... N&W cars definitely did go off line, especially to
customers requiring high grade metallurgical coal. Along the lines
of what you wrote above, I have seen pictures of C&O hoppers in Iowa
and Reading hoppers in Wisconsin. And yes, an N&W hopper in Montana
on the Milwaukee Road...
Like the C&O, the N&W had both huge eastbound and westbound volumes
of coal. It didn't all go down to Norfolk! In West Virginia along
either road it was (and is) common for loaded coal trains to pass
in opposite directions.