The NESTT mod allows you to build factories inside a train. But it doesn't allow you to nest the trains inside eachother and it is limited to the NESTT game mode.

Maybe you could add space inside trains too? The amount of space would depend on amount of wagons, and you should be able to connect the inner walls of the cargo wagons with the cargo wagon box in the outside world of the train.

Would be nice if my trains that go between stations picked up plates and delivered repairpacks to outposts without any factories needed on land.

That's a neat idea! I'm gonna have to come up with a different way to transport power into the wagons though. The wagon interiors all need to be on different surfaces, but I could add teleport areas so you can move from wagon to wagon. Entering and leaving the interior would work by riding in the wagon, then pressing a GUI button or something, because there still aren't any custom keybindings AFAIK. For power, there would be a (rather inefficient, yet uncapped) burner generator fixed inside the train wagon.

Hotkeys exist: https://mods.factorio.com/mods/Tinyboss/picker

Why do wagons have to be on different surfaces? Because it's less work? Merging and splitting surfaces you have built on might be a bit tricky I guess.

Oh, that is very convenient actually. Yes, merging and splitting surfaces would be impossibly tricky, and it really isn't worth the effort when I can just add teleport connections like with regular factory buildings, but between wagons.