Construct Tunnels through mountains for Vehicles and Trains

Have a tunnel construction capability to allow vehicles and Trains to go through mountains, etc.

due to engine limitation there is no way to make bridges or tunnels for trucks. something like no bridges in original doom.

Unity does support runtime CSG (cutting solid meshes at runtime), but it can get glitchy in a hurry. The limited angles of the terrain would make it less glitch prone.

можно к этой теме добавить тоннелепроходческие комплексы (ТПМК)
ходить он может по железной дороге( только по прямым участкам, без поворотов)
необходимо будет сделать вход в тоннель как обычный обьект из панели( по типу площадок для жд станций )
а также можно выбрать вход под тоннель для поездов и для машин

This is not a graphical problem, but a problem of land simulation, which is 2D. There is currently single height of each map point, tunnels would require more of them. All terrain simulations would became more complex. But biggest obstacle is vehicle pathfinding, which is already a big CPU hog and a reason for vehicle count limit. Trains do not use map pathfinding and navigating on rails is much more simple, so they can have bridges and unlimited amount of trains.
Bridges could be partially simulated without ability for vehicles to go under them (but still usable to cross water) as they would act as a terrain of theirs level, tunnels could be done without visuals as a building consuming vehicle and spewing it out after a while on the other side. But that would still require pathfinding changes.