Found a great solution for offline staging/storage of rolling stock... This cart from Michael's has ten drawers, each of which can hold 10-12 transition era freight cars. That's 100-120 overall! Being on wheels, it can also be wheeled easily around the train room. A layer of bubble wrap protects the cars and keeps them from jostling about.
I can't take credit for this find, though. During a recent Patreon chat for the A Modeler's Life podcast, Chris Adams provided a tour of his The Valley Local - Modeling the New Haven RR, Autumn 1948. One viewer saw such a cart in the background and asked about it. Chris has two of them. In fact, he blogged about them six years ago! I operated on Chris' layout back in January and totally missed seeing them.
I got three on sale for $29.95 each. Two for staging/storage and one for rolling stock construction projects. Versions of this solution are also available from Walmart, Amazon, and others, and prices range from $40-50. Keep on eye on the dimensions, though, as some are wider but not as deep as others.