The major hang-up is that you'll need 3D artwork to implement these vehicle types properly. Unfortunately, the game hasn't attracted much talent with those skill sets for modding.

If you want to reuse artwork in the game, it shouldn't be too hard to add these vehicle types via the mod tools. You will need a unique picture for the vehicle type, formatted into DDS format. Then you'd use the components editor to add the new vehicle type: https://wiki.gearcity.info/doku.php?id=modtools:componentseditor (tab 18 in the first picture) https://wiki.gearcity.info/doku.php?id=modtools:componentseditor#car_types

You will also need to add at least one car design in the Car Design tab. I would duplicate another vehicle from the game and set its birth and death years from 1890 to 5000, respectively.

This video loosely covers the tool: https://www.youtube.com/watch?v=B-bRHfuQBm8&list=PL1Qy-Pj_RIRtK0UuDO1h29PeuZ-FqeFCm

A full playlist is here: https://www.youtube.com/playlist?list=PL1Qy-Pj_RIRvPF48RUdW7ftj_j0gZIJfA

I haven't edited this part of the manual yet. And more/better modding videos will come someday in the future.

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That said, in the Feature Bounty System[www.ventdev.com], there are 217 new vehicle types[www.ventdev.com] waiting to funded. When the community funds any of these bounties, I'll officially add them to the game with new body artwork. Also, any vehicle-type bounties include a major overhaul of the vehicle body designer system that will improve both UX and graphical quality. So if you really want these vehicle types in the game, please consider contributing.