Just putting this idea out there, maybe someone with more LUA knowledge than me might like the idea/know how to implement it. what if a special type of couplers had the ability to connect powertrain components between two vehicles, such as a car and trailer? for example, if you had modded a combine harvester into the game (such as MRCrash's mod), where the header is a seperate vehicle from the main combine harvester, and you wanted to be able to both control the header from the harvester, and have it more realistically connected to the engine, (so the engine loads up when the header comb is spinning, etc.), you could ideally connect the attachment, and have the powertrain of the combine controlling the header. Though I'm not that experienced with how programming like this is set up, I have 2 rough ideas of how it could be implemented: 1. all of the powertrain elements of the header get added into the powertrain of the combine making one complex powertrain, (though i can guess how this may not work in a few ways). 2. a measurement of the torque/rpm going through the coupler from the combine is sent to the header, and is connected sort of like an engine to the header's powertrain. from here, it spreads to whatever diffs/shafts necessary. the only problem I see with this method is how force is applied from the header back into the combine. I could also see this being useful for things like proper air brakes on semi trailers, and perhaps some weird stuff, like an offroad trailer with driven axles. While i guess there's not that many practical applications for this, if it were reasonably easy to implement (which again, I don't know if it would be), it would be a good opportunity for another demonstration of the game's physics.
For proper stability that would require syncing the Lua VMs of the vehicles at physics step, which is way too expensive... So. I'm afraid, no this can't work properly. I'm sure you can somehow make something work, but it won't be as nice and stable as the powertrain.
