David,
Thanks for the pointers about Helicon. I've been using ricebags and mirrors to get down low, and also have a few "tabletop" or "clamp" tripods--one of which looks very much like a handgun when folded up in a camera bag. After three rather breathless, urgent airport camera bag searches, I've taken it out. Not needed for road trips!
I haven't purchased a front surface mirror as recommended by Rich Yourstone, but regular mirrors have worked OK with a little effort to eliminate reflections off the glass.
I've been using the Nikon 12-24 zoom at f/8. It's a wonderful lens for everyday shooting, but I want something sharper for the new railroad pictures--perhaps a macro? BTW, at f/22, that zoom goes from perhaps six inches to infinity, but with the diffraction you noted.
I'm still in the manual focus/manual exposure/infrared trigger stage, rather than tethering the camera to my laptop, which doesn't have Helicon installed. I get irrationally nervous tethering my camera to a computer. And the entire Helicon process is mostly painless and fast.
Besides, doing things manually allows me to think a bit during shots.