If you're me (cheap/frugal and can't wait for fancy boxes), get ahold of copy paper boxes. I followed a tip from my cousin on this.
I take paper boxes, cut them down and make my own cases. There's a basic formula here.
3" depth = Can fit 12 cars at most
4" depth = Can fit 24 cars at most.
6" depth = I have three layers of 85" boxcars, autoracks, Vert-A-Pacs and miscellaneous boxcars.
(Numbers subject to change depending on flatcars, tank cars, gons etc.)
I use sheets of bubble wrap or thin foam to keep everything safe. Worth noting here that I do not transport my super-expensive stuff this way.
I've been meaning to do a YouTube video about making these. I went "full nerd" and have assigned specific freight trains to specific, tailored cases, so my biggest case always holds my Automotive stuff. My two other larger cases hold 24 cars each which are appropriate for transcontinental manifests. One case is nothing but 24 grain hoppers, and four small ones are 12 cars of anything goes.
All said and done my case craze allows me to keep a mobile collection of cars which are safely stored and can be deployed quickly and in large numbers, all while running a different mixup of freight cars each time. These are also small enough to fit into many different styles of plastic tote, and with my totes I can get 4 single-layer cases at a time. My cousin will fit six boxes, without lids, into a larger plastic tote and haul entire trains around like this.