There is a long Youtube film, can't find it at the moment, depicting on of the Rio Turbio 30 inch gauge 2-10-2s being operated by a single crewman. These coal burners also had mechanical stoking, so its not like the engineer had to stop and shovel coal. There is also a terrifying segment, to me at least, where the crewman walked out on the walkway (my mind just went blank as to the correct term) along the firerman's side of the boiler and bent down to check the cylinder. If I was the cameraman I would have been worried.
I guess this was possible because the Rio Turbio is basically a straight railway out in open land. You can see for miles and miles what is approaching. I would never want to try and have a single crewman operate alone on a mountain railway like the D&S or the C&TS or anything similar.
I don't know if this was common practice or the fireman was the cameraman and the engineer handled everything for a bit so they could have a video.