As with many ND hobbies. It sprawls out in almost as many ways as people who do it.

Some love the massive powerful machinery that is a locomotive. Others are obsessed with the timings and predictability. Still others love the organisational side of things.

It even overlaps with the model train interest groups. Recreating aspects of the train network, based on their own interests.

ND hobbyists are a bit like cats in many ways. There's not much pack drive. We tend to wander and explore interests in our own way, independent of the labels that get applied. It can be broadly grouped, but has a lot of spread.