Hey all - thanks for the insightful responses~! Especially based on these comments, I believe I will continue to focus on a variety -- Class 1s and branchlines/shortlines (the rural one-a-day lines have always been the most interesting to me) -- but narrow the focus tighter geographically. These will all be original shots, that's why I thought the SP in particular would have a strong audience -- as I have a hoard of 1980s-1990s SP I am trying to narrow down.
I appreciated the Don Ball references - one of his books was actually the first RR book I ever bought, back in the early 1980s. As with Ball, I always have tried to provide a mixture of images from more than one RR; my leanings are to freights and to the "backwater" lines; and I have focused on the Northwest and Midwest... kind of a "northern tier" approach, because that represents where I've lived and where I'm from... THANKS again!