Since there are limitations on how good the groups search engine works, I've found a good way to find related groups is to join a group I'm interested in and then search the LINKS area. While some groups have no links, others have many links to other groups and to other web sites I've never heard of before that have proven beneficial. I've managed to join several railroad related Yahoo groups, and one of my favorites is not currently active but has a large photo album that is very interesting to me as it is photos from where I grew up.