I shot family, vacations, and the occasional train with a D50 and a 18-200 for ten years. Used that combination to shoot at Chama last year, seen here:
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glenn.pulpitrock.net]
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glenn.pulpitrock.net]
6MP resolution with the 18-200 worked fine, but I found myself sharpening quite a bit in post-processing. I also wanted more dynamic range; you get a lot of shadow in the interesting parts of train pictures, and drawing that out was challenging with the D50 sensor. So, I bought a D7000 on closeout and I'm using it with the 18-200. The extra couple of stops of sensor range make the difference I was looking for, but the camera clearly outclasses the lens in sharpness. I used that combo to shoot this year's Chama Steam Fall Classic; don't have an album yet, but here's one image:
18mm:
200mm (I think, can't see exif):
If I had a few extra dollars, I'd go get the 18-140. As pointed out previously, it's a good bit sharper than the 18-200.
On the other topics brought out above:
I'm a one-lens guy, also. Walking about the yard following the locomotives, I like having all my focal lengths available with a twist, and I feel it's safer than having to concentrate on lens-changing while standing in an industrial setting.
I do focal-length stats after my shooting, and I find that the majority of my shots are either at 18mm or at 200mm; my use of telephoto is not about reach, it's about compressed perspective, and I think most focal lengths above 135mm would do it for me. Now, wildlife shooting, particularly birds, won't be good with any focal length under 300mm, IMHO. These are all DX...
Oh, and the dust thing is a significant consideration, too.