After reading through the ideas so far, all excellent, here are some if the ideas I have formulated.
1. A picture of anything. A locomotive must be included, but it can not be the central subject.
2. People who make the railroads run.
3. Items of infrastructure, which are small enough to fit into a 40-foot boxcar without being dismantled.
4. What do, or did, the look like on the inside?
In all cases, creative presentation is a must. Normal "darkroom" manipulation is OK, such as cropping, or adjusting brightness or contrast.