For the past few months, I have been working on overhauling, lining up, and adding to relationships all the New Mexico railroads. I am about 90% of the way to my goal. Also, I noticed some of the other states and countries have railroad pages on the wiki, so I went ahead and started one for the New Mexico Railways.
Some lessons learned:
Railroads often have many more bridges than streets (I suppose since they often intersect land parcels).
service=siding actually exists (was marking them as spurs up until a few months ago, but went back and fixed the other ones).
Many spurs exist on the main lines, and it is often hard to determine if the company or the railway maintains it (for now, I excluded them from the subdivision relationships).
It's difficult to keep an ordered list of ways on a relation. For example, I would find one of two lines within a relationship not in the relationship. It's difficult to pinpoint exactly where each segment lines up with the segment in the other main line.
There is a map that uses OSM data to render an explorable map of railroads.
If you ever need a break from the commotion of mapping large urban areas, there are still plenty of railroad segments between cities that haven't been touched since the TIGER import! Happy Mapping!