Many years ago, you decided to create that train layout that you dreamed of as a youngster. You settled on a theme and started acquiring engines and rolling stock that seemed relevant and looked good. But your house wasn't as big as your vision, so the O gauge items went to storage and a small HO or N layout was built to satisfy the craving. The years went by and the houses grew, but a road warrior job and a boat for half the year largely precluded basement finishing and layout construction. Meanwhile, the O gauge item inventory kept growing, sometimes duplicating itself.
Retirement solved the road warrior job problem and a new home with a large auxiliary building solved most space concerns, so the layout design was finalized and construction began. Three winters of construction work later, the benchwork is finished, the roadbed and track laid, and the wiring has been completed. Although the layout is far from done - track ballasting & painting, engine facility benchwork & trackage, and scenery projects lie ahead - O gauge trains can be made up and run continuously, if one desires, for the first time since the carpet or plywood sheet layout of one's early years.
So you look at all those boxes stowed under the layout and ask yourself, "what should be the first trains that I run after a functional test?" Do you meekly run the least of your inventory, since it's an incomplete railroad? Get out the finest locomotive you own and the fanciest cars for it to pull, because you've waited long enough? Or just grab whatever's atop the stacks and easiest to unpack? (And wondering if you subconsciously slowed construction to avoid this decision.)