I think the key is setting parameters and honoring them, because a collection of anything can reach a “where did all of this come from?” state overnight.
When I got back into it about 5 years ago I looked at the collection my dad and I had assembled, which was about 2/3 MPC and the balance Postwar and LTI. Admitting to myself that I am a completist by nature, that I have two kids to put through college, and the nostalgic pull ‘70s and ‘80s Lionel has for me, I decided to collect only MPC going forward. Being a finite, closed era I figured I can reach a point that I have all of it I could want at some point.
I also set limits on where the trains can be stored in the house—one part of the basement. Once that area is completely full, something will have go before I add something new. It’s getting crowded so I’ve got a couple dozen LTI pieces queued up for eBay, as they no longer fit in with the rest of the collection.