I'm mostly interested in metro/subways, but also over the ground systems and rapid transit in general.
Mostly the vehicles themselves, the rails, and the related technologies. I mean, I do enjoy the city planning bits, but some channels focus too much on the design and evolution of the lines and I don't like that, looking at a map is not really enticing to me. A healthy mix would be nice, some engineering deep dives mixed with history, urban planning, and a lot of train porn! (Not rule 34 lol).