Here are photos of two boxes I use for transporting 0 gauge trains to and from our toy train museum operating sessions, plus another one I set up for a specific Standard Gauge locomotive.
This is a Sterilite box I got at Target. As you can see it will hold six 40-50 foot freight cars. If the cars are 40-footers or less, you can get an extra car in on the side as I have done with covered hoppers in this photo. There is enough headroom that you could put gondolas or flatcars on top, or there might be enough room for two layers with the cars laid on their sides. I wouldn't do that with scale cars but it would be fine with 6464 boxcars or something like that. I like these because they have latches on the lids, they stack well and will stay stacked in the back of a pickup, and they have lots of grab space at the ends. Overall size is about 24x18 and usable interior space is about 19x15. If I remember correctly they cost six or seven dollars, less if you get them on sale.
I got these boxes at York, and then a few more later direct from the seller in California. Uppa Valley Lines doesn't have a web site, but I'm pretty sure they are still in business. You can phone them to order. They also have extra dividers if you want to reconfigure the interior. I don't remember exactly how much the boxes cost; I think it was around 15 bucks per box. They are nice for storing and transporting standard size cars and they are super easy to get the cars in and out of. I store a couple of dozen Weaver cars in them so they are ready to go at any time.
This is how I transport a Standard Gauge Super 381 to and from the museum, and I also store the locomotive in this box at home. The original box for a Super 381 is a monstrous thing, with a metal cage that you bolt the unit into and then put the cage in a styrofoam enclosure, then the actual box. It takes at least 15 minutes to get it in or out of the box. So, I found a heavy cardboard box that fit the Super, then used some leftover wood blocks to hold it in place so it wouldn't move around. For this particular unit that works better than bubble wrap or other soft padding. The setup is very secure and it gets the Super around town without damage.