I found the following:

https://mods.factorio.com/mods/AndrolGenhald/boxing-2
https://mods.factorio.com/mods/siikk/resource-compression
Both of these increase density of storage in trains / chests because the compressed items stack. Mine do not.

https://mods.factorio.com/mods/Neel ...

Type: Mod
Name: Boxes
Description: Adds beltable boxes to the game.
License: MIT
Version: 0.1.0
Release: 2018-01-06
Tested-With-Factorio-Version: 0.16.15
Category: SimpleExtension
Tags: Free, Logistics
Mod Portal Link: https://mods.factorio.com/mods/paradigmsort/boxes


Long description
Boxes ...

That's reasonable.

I actually came across this when trying to create a mod which would add "bins" which are basically chests which can sent along belts, but don't overflow the sides like cars do.

As far as I know vehicles are the only thing that can be placed on top of belts like this, so I guess ...

What I did:
- Create a new sandbox game with no mods (default map generation, all techs, cheat mode, always day)
- Place the following:
a) car on grass -> inserter -> chest
b) car on belt -> inserter -> chest
c) car on belt -> space -> inserter -> chest
Screen Shot 2017-11-12 at 9.51.40 AM.png ...