Tractor Trailers

I've seen semi-trucks suggest previously, but I want to flesh the idea out a bit more, slash give a different perspective. Semi trucks would be a side grade for the current haul trucks, and an upgrade for the rigid T2 trucks, currently in game. They would be sort of a hybirdization of trucks and trains. Just like trains, the tractor would have no capacity of it's own besides fuel, instead the player attachs a trailer to it durring construction, which functions as the vehicle's transportation capacity. Additionally, these trailers can be swaped out, upgrading a tractor trailer combination, with a different trailer, will send the truck back to the depot, where it's trailer is removed, and a new one constructed, the old trailer can be optionally dismantled, or retained, for later use on another vehicle. If there is a free trailer available it will be used for the upgrade, instead of building a new one. These would be a more flexible intermediate solution, compared to haul trucks.

unnecessary complication, destroys current balance, removes the idea or trains - do I need to continue ?

No more complicated than needing to replace a tank Haul Truck with a Dumper, and can be done via the upgrade/replace tool. Fail to see how it "destroyes current balance", it would sit between haul trucks and trucks in terms of capacity, there is also no existing "flatbed" haul truck equivalent. Trains would still be more efficient in terms of maintainence and fuel costs, this is not mutually exclusive to the existance of trains. Trains can't go everywhere, that's where these come in, and now with T2 trains (yes, writing this after U4 is probably a bit unfair) these would still move less material than a train.