Judging by postings on the Forum, the Lionel Substation and Municipal Building kits are probably the most kitbashed buildings in O gauge. So here's my contribution.
Several years ago I needed a freight warehouse on my home layout so I took three substation and one municipal building kits and put them together end to end. The unexpected acquisition of a custom built structure which wound up going where I intended the warehouse to go, shelved the project for a long time. I finally realized that it would be a good fit on our club layout, The River City 3 Railers, so I finished it. Also Miller Engineering came out with a sign for The Acme Corporation (you know the folks that made all the ridiculous props and explosives for the Road Runner cartoons) which was a perfect theme for the warehouse.
So this warehouse is a distribution facility for the main Acme factory which is on my home layout.
Ken