Everything is too cheap, too easy.
The carts consumes very low amounts of fuel and costs almost nothing for the job he does.
Also, more tiers of carts would be nice, something like:
cart mk1 = 3 inventory slots
cart mk2 = 6
cart mk3 = 10

So, better balancing. Will do.

I think mark 1/2/3 style upgrades are lazy game design, though. Tiers should have real differences and trade offs. Currently there is:

Fuel burning with long range
Electric with much shorter range and slower charge
... need more ...

I made the mods TA miners, Wind Generators and Heavy tanks, all have tiers, but they grow in size and have different and challenging recipes.
The problem if your mod is that you have little space to work, i mean, 1x1 tile is a very small space.
If you manage to make a 2x2 mechanic, it will be a lot easier to come with ideas and models. And speaking of models, yours is not bad, but i can make new ones if you like, just send here a picture or sketch that you have in mind. (my latest work is the TA Miners, check it out to see my current level of work)

Heh.. thanks, my model is a bit of hack :) Angel offered to do some graphics at some stage too. I want to be sure the POC is worth while before taking up anyone's time.

Started the rebalancing in 0.1.11 with slightly more expensive recipes, worse fuel consumption for burner carts, and a reduction in speed to 5km/h. Haven't looked at inventory sizes yet.