Gavin, I'd actually say that coverage of most US roads that lasted into the post-war era is pretty good. Coverage of roads that hired photographers to promotive them (e.g. William Henry Jackson on the DSP&P) is also quite good.
BUT...
These were the minority. I am aware of ONE photo of a TC&Stl freight car. This is significant, because the TC&Stl was one of the largest and historically most significant of all narrow gauge railroads. The basic coverage and locomotive roster were researched and published by John Rehor decades ago.
Many railroads don't have anywhere close to as much published on them as the TC&Stl (which has never had anything published on it in the Narrow Gauge Gazette, Model Railroader, Trains, etc). I've searched extensively for info on the Cincinnati & Eastern...having found 4-5 photos and about a brochure's worth of info...many roads have fewer photos and only a couple paragraphs published on them. I'm aware of 20 narrow gauge roads in my home state. Our neighbors to the east had around 25 in their state. Most narrow gauge fans would be hard pressed to name five of these 45 roads.
Good resources in the states are libraries and historical societies. For the TC&Stl, a researcher would visit Bowling Green State University and Wright State University libraries. The Allen County Museum in Lima would be a necessary stop as well (same Lima as Shays/Lima Locomotive Works). The Delphos newspaper(s) are available through the universities (IIRC). The Ohio Historical Society would have some goodies too. Little gems could be gleamed from the trade journals of the day (e.g. Railway Age). Since the road is 125 years in the past, odds of finding surviving employees is slim...or really anyone except maybe their now elderly grandkids.
Michael