I have trains in both HO and N gauges and from different periods and countries as I haven't been sufficiently disciplined with myself and just bought what took my liking. I'm not even sure how much I have in total but the collection has been growing slowly over the years, with the earliest items going back to my childhood.
I guess at some point I will have to go through it and sell the various items that don't interest me anymore.
I don't have any fixed layout but have a big box of track parts and when I take a fancy to running some trains I will clip together something ad-hoc. Especially when i need to cheer myself up. So I will do anything from a huge oval to just sit back and watch it run, to complicated switching yards that require a lot of hands on operation.
I try not to mix different periods or countries though within one operating session, although sometimes it will happen.
I want to build a proper layout with scenery etc eventually. It's really a question of finding the time, and also of planning it properly. I have sketched many layouts on paper, but was never really totally satisfied.